$B    117    452 


ommon 


als  ana (, 


CROSBY" 


/B  E  R  K  E  L  E  Y 
* 


UNIVERSITY   OF 
V      CALIFORNIA 


r 


of  Natural  ^i 


GUIDES  FOR  SCIENCE-TEACHING 


No.  XII 


COMMON  MINERALS  AND  ROCKS 


BY  WILLIAM  O.  CROSBY 


D.  C.  HEATH  &  CO.,   PUBLISHERS 
BOSTON    NEW  YORK    CHICAGO 


Copyright 

BY  THE  BOSTON  SOCIETY  OF  NATURAL  HISTORY 
1881 

I    D  2 


INTRODUCTION. 


MINERALS  and  rocks,  or  the  inorganic  portions  of 
the  earth,  constitute  the  proper  field  or  subject-matter 
of  the  science  of  Geology.  Now  the  inorganic  earth, 
like  an  animal  or  plant,  may  be  and  is  studied  in  three 
quite  distinct  ways,  giving  rise  to  three  great  divisions 
of  geology,  which,  as  will  be  seen,  correspond  closely 
to  the  main  divisions  of  Biology. 

First,  we  may  study  the  forces  now  operating  upon 
and  in  the  earth  —  the  geological  agencies — such  as 
the  ocean  and  atmosphere,  rivers,  rain  and  frosts, 
earthquakes,  volcanoes,  hot  springs,  etc.,  and  observe 
the  various  effects  which  they  produce.  We  are  con- 
cerned here  with  the  dynamics  of  the  earth ;  and  this 
is  the  great  division  of  dynamical  geology,  correspond- 
ing to  physiology  among  the  biological  sciences. 

Or,  second,  instead  of  geological  causes,  we  may 
study  more  particularly  geological  effects,  observing 
the  different  kinds  of  rocks  and  of  rock-structure  pro- 
duced by  the  geological  agencies,  not  only  at  the 
present  time,  but  also  during  past  ages.  This  method 
of  study  gives  us  the  important  division  of  structural 
geology,  corresponding  to  anatomy  and  morphology. 


6  INTRODUCTION. 

All  phenomena  present  two  distinct  and  opposite 
aspects  or  phases  which  we  call  cause  and  effect;  and 
so  in  dynamical  and  structural  geology  we  are  really 
studying  the  opposite  sides  of  essentially  the  same 
classes  of  phenomena.  In  the  first  Division  we  study 
the  causes  now  in  operation  and  observe  their  •  -effects  ; 
and  then,  guided  by  the  light  of  the  experience  thus 
gained,  we  turn  to  the  effects  produced  in  the  past  and 
seek  to  refer  them  to  their  causes. 

These  two  divisions  together  constitute  what  is 
properly  known  as  physiography ;  and  they  are  both 
subordinate  to  the  third  great  division  of  geology, — 
historical  geology,  —  which  corresponds  to  embry- 
ology. 

The  great  object  of  the  geologist  is,  by  studying 
the  geological  formations  in  regular  order,  from  the 
oldest  up  to  the  newest,  to  work  out,  in  their  proper 
sequence,  the  events  which  constitute  the  earth's 
history ;  and  dynamical  and  structural  geology  are 
merely  introductory  chapters,  the  alphabet,  as  it  were, 
which  must  be  learned  before  we  are  prepared  to  read 
understandingly  the  grand  story  of  the  geological 
record. 

Our  work  in  this  short  course  will  be  limited  to  the 
first  two  divisions,  —  i.e.,  to  dynamical  and  structural 
geology-  We  will  attempt,  first,  a  general  sketch  of 
the  forces  now  concerned  in  the  formation  of  rocks 
and  rock-structures ;  and  after  that  we  will  study  the 
composition  and  other  characteristics  of  the  common 
minerals  and  rocks. 

The  scope  of  this  work,  and  its  relations  to  the  whole 
field  of  geology,  are  more  clearly  indicated  by  the  fol- 
lowing classification  of  the  geological  sciences  :  — 


GEOLOGY 


INTRODUCTION. 

'DYNAMICAL  GEOLOGY  (P^y^l  Geology. 
I  Chemical  Geology. 

{Mineralogy. 
I  Petrology. 
.HISTORICAL  GEOLOGY. 


Many  teachers  will  desire  to  fill  in  some  of  the  details 
of  the  outline  sketch  presented  in  this  Guide,  and  for 
this  purpose  the  following  works  are  especially  recom- 
mended :  — 

ELEMENTS  OF  GEOLOGY.     By  Prof.  Joseph  LeConte.      1882. 

D.  Appleton  &  Co.,  New  York.     Nearly  600  pages. 
MANUAL  OF  GEOLOGY.    By  Prof.  J.  D.  Dana.    Third  edition. 

1880.     800  pages. 
TEXT-BOOK  OF  GEOLOGY.    By-  Prof.  A.  Geikie.     1882.    Mao 

millan  &  Co.»  London.     Nearly  looo  pages. 

As  a  reference-book  for  mineralogy,  the  following 
treatise  is  unsurpassed  :  — 

TEXT-BOOK  OF  MINERALOGY.     By   Edward  S.   Dana.     1883. 
John  Wiley  &  Sons,  New  York. 

And,  as  an  introduction  to  the  study  of  minerals, 
and,  through  these,  to  the  study  of  rocks,  — 

FIRST  LESSONS  IN  MINERALS.    Science  Guide  No.  XIII.    By 
Mrs.  E.  H.  Richards. 

cannot  be  too  highly  recommended.  Teachers  will 
find  this  little  primer  of  46  pages  invaluable  with  young 
children,  and  with  all  who  have  had  no  previous  train- 
ing in  chemistry. 

As  an  admirable  continuation  of  the  work  begun  in 
these  pages,  teachers  are  referred  to  Professor  Shaler's 
"  First  Book  in  Geology."  In  this  our  brief  sketch  of 


8  1NTR  OD  UC  TION. 

the  geological  agencies  is  amplified  and  beautifully 
illustrated;  and  rarely  have  the  wonderful  stories  of 
the  river,  ocean-beach,  glacier,  and  volcano  been  told 
so  effectively.  In  the  chapter  on  the  history  of  life  on 
the  globe  the  main  outlines  of  historical  geology  are 
skillfully  brought  within  the  comprehension  of  begin- 
ners. The  directions  to  teachers  are  fully  in  accord 
with  the  modern  methods  and  ideas,  and  are  a  very 
valuable  feature  of  the  book. 


DYNAMICAL    GEOLOGY. 


WHEN  we  think  of  the  ocean  with  its  waves,  tides, 
and  currents,  of  the  winds,  and  of  the  rain  and  snow, 
and  the  vast  net-work  of  rivers  to  which  they  give  rise, 
we  realize  that  the  energy  or  force  manifested  upon  the 
earth's  surface  resides  chiefly  in  the  air  and  water — 
in  the  earth's  fluid  envelope  and  not  in  its  solid  crust. 
And  it  would  be  an  easy  matter  to  show  that,  with  the 
exception  of  the  tidal  waves  and  currents,  which  of  course 
are  due  chiefly  to  the  attraction  of  the  moon,  nearly  all 
this  energy  is  merely  the  transformed  heat  of  the  sun. 
Now  the  air  and  water  are  two  great  geological  agen- 
cies, and  therefore  the  geological  effects  which  they 
produce  are  traceable  back  to  the  sun. 

Organic  matter  is  another  important  geological  agent ; 
but  all  are  familiar  with  the  generalization  that  connects 
the  energy  exhibited  by  every  form  of  life  with  the  sun  ; 
and,  besides,  it  is  scarcely  necessary  to  allude  to  the  ob- 
vious fact  that  all  animals  and  plants,  so  far  at  least  as 
any  display  of  energy  is  concerned,  are  merely  differ- 
entiated portions  of  the  earth's  fluid  envelope.  And  so, 
if  space  permitted,  it  might  be  shown  that,  with  the 
exception  of  the  tides,  nearly  every  form  of  force 


10  DYNAMICAL    GEOLOGY. 

manifested  upon  the  earth's  surface  has  its  origin  in 
the  sun. 

Of  this  trio  of  geological  agencies  operating  upon 
the  earth's  surface  and  vitalized  by  the  sun  —  water, 
air,  and  organic  matter  —  the  water  is  by  far  the  most 
important,  and  so  it  is  common  to  call  these  collec- 
tively the  aqueous  agencies.  Hence  we  have  solar 
agencies  and  aqueous  agencies  as  synonymous  terms. 

The  aqueous  agencies  include,  on  one  side,  air  and 
water,  or  inorganic  agencies  ;  and,  on  the  other,  animals 
and  plants,  or  organic  agencies. 

Let  us  notice  briefly  the  operation  of  these,  begin- 
ning with  the  air  and  water. 


I.    AQUEOUS  AGENCIES. 
i.  Air  and  Water,  or  Inorganic  Agencies. 

CHEMICAL  EROSION.  —  Attention  is  invited  first  to  the 
specimens  numbered  i,  2,  3,  and  4.  No.  i  is  a  sound, 
fresh  piece  of  the  rather  common  rock,  diabase ;  and 
those  who  are  acquainted  with  minerals  will  recognize 
that  the  light-colored  grains  in  the  rock  are  feldspar, 
and  the  dark,  augite.  This  specimen  came  from  a 
depth  in  the  quarry,  and  has  not  been  exposed  to  the 
action  of  the  weather. 

The  second  specimen  differs  from  the  first,  appar- 
ently, as  much  as  possible ;  and  yet,  except  in  being 
somewhat  finer  grained,  it  was  originally  of  precisely 
similar  composition  and  appearance.  In  fact,  it  is  a 
portion  of  the  same  rock,  but  a  weathered  portion.  In 
this  we  can  no  longer  recognize  the  feldspar  and  augite 


AQUEOUS  AGENCIES.  II 

as  such,  but  both  these  minerals  are  very  much  changed, 
while  in  the  place  of  a  strong,  hard  rock  we  have  an 
incoherent  friable  mass,  which  is,  externally  at  least, 
easily  crushed  to  powder ;  and  with  the  next  step  in 
the  weathering,  as  we  may  readily  observe  in  the 
natural  ledges,  the  rock  is  completely  disintegrated, 
forming  a  loose  earth  or  soil. 

We  have  two  examples  of  such  natural  powders  in 
the  specimens  numbered  3  and  4 ;  and  by  washing 
these  (especially  the  finer  one,  No.  4)  with  water,  we 
can  prove  that  they  consist  of  an  impalpable  substance 
which  we  may  call  clay,  and  angular  grains  which  we 
may  call  sand.  The  sand-grains  are  really  portions 
of  the  feldspar  not  yet  entirely  changed  to  clay. 

Thus  we  learn  that  the  result  of  the  exposure  of  this 
hard  rock  to  the  weather  is  that  it  is  reduced  to  the 
condition  of  sand  and  clay.  What  we  mean  especially 
by  the  weather  are  moisture  and  certain  constituents 
of  the  air,  particularly  carbon  dioxide. 

The  action  of  the  weather  on  the  rocks  is  almost  en- 
tirely chemical.  With  a  very  few  exceptions,  the  prin- 
cipal minerals  of  which  rocks  are  composed,  such  as 
feldspar,  hornblende,  augite,  and  mica,  are  silicates,  i.e., 
consist  of  silicic  acid  or  silica  combined  with  various 
bases,  especially  aluminum,  magnesium,  iron,  calcium, 
potassium,  and  sodium. 

Now  the  silica  does  not  hold  all  these  bases  with 
equal  strength;  but  carbon  dioxide,  in  the  presence 
of  moisture,  is  able  to  take  the  sodium,  potassium, 
calcium,  and  magnesium  away  from  the  silica  in  the 
form  of  carbonates,  which,  being  soluble,  are  carried 
away  by  the  rain-water. 


12  DYNAMICAL    GEOLOGY. 

The  silicate  of  aluminum,  with  more  or  less  iron, 
takes  on  water  at  the  same  time,  and  remains  behind 
as  a  soft,  impalpable  powder,  which  is  common  clay. 

In  the  case  of  our  diabase,  contined  exposure  to  the 
weather  would  reduce  the  whole  mass  to  clay.  But 
other  rocks  contain  grains  of  quartz,  a  hard  mineral 
which  cannot  be  decomposed,  and  it  always  forms  sand. 
Certain  classes  of  rocks,  too,  such  as  the  limestones 
and  some  iron-ores,  are  completely  dissolved  by  water 
holding  carbon  dioxide  in  solution,  and  nothing  is  left 
to  form  soil,  except  usually  a  small  proportion  of  insol- 
uble impurities  like  sand  or  clay. 

Let  us  see  next  how  these  agents  of  decay  get  at 
the  rocks.  Neither  water  nor  air  can  penetrate  the 
solid  rock  or  mineral  to  any  considerable  extent,  so 
that  practically  the  action  is  limited  to  surfaces,  and 
whatever  multiplies  surfaces  must  favor  decomposition. 

First,  we  have  the  upper  surface  of  the  rock  where 
it  is  bare,  but  more  especially  where  it  is  covered  with 
soil,  for  there  it  is  always  wet. 

All  rocks  are  naturally  divided  by  joints  into  blocks, 
which  are  frequently  more  or  less  regular,  and  often  of 
quite  small  size.  Water  and  air  penetrate  into  these 
cracks  and  decompose  the  surfaces  of  the  blocks,  and 
thus  the  field  of  their  operations  is  enormously  ex- 
tended. These  rock-blocks  sometimes  show  very 
beautifully  the  progress  of  the  decomposing  agents 
from  the  outside  inward  by  concentric  layers  or  shells 
of  rotten  material,  which,  in  the  larger  blocksv  pften 
envelop  a  nucleus  of  the  unaltered  rock. 

It  is  interesting  to  observe,  too,  that  these  concentric 
lines  of  decay  cut  off  the  angles  of  the  original  blocks* 


AQUEOUS  AGENCIES.  13 

so  that  the  undecomposed  nucleus,  when  it  is  found, 
is  approximately  spherical  instead  of  cuboidal.  Both 
these  points  are  well  illustrated  by  specimen  No.  2  ; 
for  although  now  nearly  spherical,  it  was  originally 
perfectly  angular,  and  has  become  rounded  by  the 
peeling  off,  in  concentric  layers,  of  the  decomposed 
material,  and  in  most  cases  several  of  these  layers  are 
distinctly  visible,  like  the  coats  of  an  onion.  But  by 
stripping  these  off  we  should  discover,  in  all  the  larger 
balls  at  least,  a  solid,  spheroidal  nucleus,  while  in  the 
smaller  balls  the  decomposition  has  penetrated  to  the 
centre. 

In  the  rocks  also  we  find  many  imperfect  joints  and 
minute  cracks.  In  cold  countries  these  are  extended 
and  widened  by  the  expansive  power  of  freezing  water, 
and  thus  the  surfaces  of  decomposition  become  con- 
stantly greater. 

Nearly  all  rocks  suffer  this  chemical  decomposition 
when  exposed  to  the  weather,  but  in  some  the  decay 
goes  on  much  faster  than  in  others.  Diabase  is  one  of 
the  rocks  which  decay  most  readily ;  while  granite  is, 
among  common  rocks,  one  of  those  that  resist  decay 
most  effectually. 

The  caverns  which  are  so  large  and  numerous  in 
most  limestone  countries  are  a  splendid  example  of 
the  solvent  action  of  meteoric  waters,  being  formed 
entirely  by  the  dissolving  out  of  the  limestone  by  the 
water  circulating  through  the  joint  cracks.  The  pro- 
cess must  go  on  with  extreme  slowness  at  first,  when 
the  joints  are  narrow,  and  more  rapidly  as  they  are 
widened  and  more  water  is  admitted.  We  get  some 
idea,  too,  of  the  magnitude  of  the  results  accomplished 


14  DYNAMICAL    GEOLOGY. 

by  these  silent  and  unobtrusive  agencies  when  we  re- 
fleet  that  almost  all  the  loose  earth  and  soil  covering 
the  solid  rocks  are  simply  the  insoluble  residue  which 
carbon  dioxide  and  water  cannot  remove.  In  low  lati- 
tudes, where  a  warm  climate  accelerates  the  decay  of 
the  rocks,  the  soil  is  usually  from  50  to  300  feet  deep,, 

MECHANICAL  EROSION.  —  On  the  edge  of  the  land.  — 
Let  us  trace  next  the  mechanical  action  of  water  and 
air  upon  the  land.  First  we  will  consider  the  edge  of 
the  land,  where  it  is  washed  by  the  waves  of  the  sea. 
Whoever  has  been  on  the  shore  must  have  noticed 
that  the  sand  along  the  water's  edge  is  kept  in  con- 
stant motion  by  the  ebb  and  flow  of  the  surf. 

Where  the  beach  is  composed  of  gravel  or  shingle 
the  motion  is  evident  to  the  ear  as  well  as  the  eye ; 
and  when  the  surf  is  strong,  the  rattling  and  grinding 
of  the  pebbles  as  they  are  rolled  up  and  down  the 
beach  develops  into  a  roar. 

The  constant  shifting  of  the  grains  of  sand,  pebbles, 
and  stones  is,  of  course,  attended  by  innumerable  col- 
lisions, which  are  the  cause  of  the  noise.  Now  it  is 
practically  impossible,  as  we  may  easily  prove  by  ex- 
periment, to  knock  or  rub  two  pieces  of  stone  together, 
at  least  so  as  to  produce  much  noise,  without  abrading 
their  surfaces ;  small  particles  are  detached,  and  sand 
and  dust  are  formed. 

That  this  abrasion  actually  occurs  in  the  case  of  the 
moving  sand  is  most  beautifully  shown  by  the  sand-- 
blast. We  are  to  conclude,  then,  that  every  time  a 
pebble,  large  or  small,  is  rolled  up  or  down  the  beach 
it  becomes  smaller,  and  some  sand  and  dust  or  clay 
are  formed  which  are  carried  off  by  the  water. 


AQUEOUS  AGENCIES.  15 

But  what  are  the  pebbles  originally?  This  question 
is  not  difficult.  A  little  observation  on  the  beach  shows 
us  that  the  pebbles  are  not  all  equally  round  and  smooth, 
but  many  are  more  or  less  angular.  And  we  soon  see 
that  it  is  possible  to  select  a  series  showing  all  grada- 
tions between  the  most  perfectly  rounded  forms  and 
angular  fragments  of  rock  that  are  only  slightly  abraded 
on  the  corners.  The  three  principal  members  of  such  a 
series  are  shown  in  specimens  5,  6,  and  7  from  the 
beach  on  Marblehead  Neck ;  but  equally  instructive 
specimens  can  be  obtained  at  many  other  points  on 
our  coast.  It  is  also  observable  that  the  well-rounded 
pebbles  are  much  smaller  on  the  average  than  the  angu- 
lar blocks. 

From  these  facts  we  draw  the  legitimate  inference 
that  the  pebbles  were  all  originally  angular,  and  that 
the  same  abrasion  which  diminishes  their  size  makes 
them  round  and  smooth. 

A  little  reflection,  too,  shows  that  the  rounding  of 
the  angular  fragments  is  a  natural  and  necessary  result 
of  their  mutual  collisions  ;  for  the  angles  are  at  the  same 
time  their  weakest  and  most  exposed  points,  and  must 
wear  off  faster  than  the  flat  or  concave  surfaces. 

Having  traced  each  pebble  back  to  a  larger  angular 
rock-fragment,  the  question  arises,  Whence  come  these 
angular  blocks  ? 

Behind  our  gravel-beach,  or  at  its  end,  we  have 
usually  a  cliff  of  rocks.  As  we  approach  this  it  is  dis- 
tinctly observable  that  the  angular  pebbles  are  more 
numerous,  larger,  and  more  angular;  and  a  little 
observation  shows  that  these  are  simply  the  blocks 
produced  by  jointing,  and  that  the  cliff  is  entirely 


1 6  DYNAMICAL    GEOLOGY. 

composed  of  them.  In  other  words,  our  cliff  is  a  mass 
of  natural  masonry,  which  chemical  agencies,  the  frost, 
and  the  sea  are  gradually  disintegrating  and  removing. 
As  soon  as  the  blocks  are  brought  within  reach  of  the 
surf  their  mutual  collisions  make  them  rounder  and 
smaller ;  and  small  round  pebbles,  sand,  and  clay  are 
the  final  result. 

For  a  more  complete  account  of  the  formation  of 
pebbles,  teachers  are  referred  to  the  first  or  introduc- 
tory number  of  this  series  of  guides,  by  Prof.'  Hyatt, 
"About  Pebbles." 

Where  the  waves  can  drive  the  shingle  directly 
against  the  base  of  the  cliff,  this  is  gradually  ground 
away  in  the  same  manner  as  the  loose  stones  them- 
selves, sometimes  forming  a  cavern  of  considerable 
depth,  but  always  leaving  a  smooth,  hard  surface, 
which  is  very  characteristic,  and  contrasts  strongly 
with  the  upper  portion  of  the  cliff,  which  is  acted 
on  only  by  the  rain  and  frost.  A  good  example 
of  such  a  pebble-carved  cliff  may  be  seen  behind  the 
beach  on  the  sea-ward  side  of  Marblehead  Neck. 

The  sea  acts  within  very  narrow  limits  vertically,  a 
few  feet  or  a  few  yards  at  most ;  but  the  coast-lines  of 
the  globe  (including  inland  lakes  and  seas)  have  an 
aggregate  length  of  more  than  150,000  miles.  Hence 
it  is  easy  to  see  that  the  amount  of  solid  rock  ground 
to  powder  in  the  mill  of  the  ocean-beach  annually 
must  be  very  considerable. 

MECHANICAL  EROSION.  —  On  the  surface  of  the 
land.  —  I  next  ask  attention  to  the  mechanical  action 
of  water  upon  the  surface  of  the  land. 

It  is  a  familiar  fact  that  after  heavy  rains  the  road- 


AQUEOUS  AGENCIES.  17 

side  rills  carry  along  much  sanfl  and  clay  (which  we 
know  have  been  produced  by  the  previous  action  of 
chemical  forces),  and  also  frequently  small  pebbles  01 
gravel.  It  is  easy  to  show  that  in  all  important  respects 
the  rill  differs  in  size  only  from  brooks  and  rivers ;  and 
the  former  afford  us  fine  models  of  the  systems  of  val- 
leys worn  out  during  the  lapse  of  ages  by  rivers.  The 
turbidity  of  rivers  is  often  very  evident,  and  in  shallow 
streams  we  can  sometimes  see  the  pebbles  rolled  along 
by  the  current. 

Now  here,  just  as  on  the  beach,  the  collisions  of 
rock-fragments  are  attended  by  mutual  abrasion,  sand 
and  clay  are  formed,  and  the  fragments  become  smaller 
and  rounder.  Our  series  of  pebbles  from  the  beach 
might  be  matched  perfectly  among  the  river-gravel. 
In  mountain  streams  especially  we  may  often  observe 
that  pebbles  of  a  particular  kind  of  rock  become  more 
numerous,  larger,  and  more  angular  as  we  proceed  up 
stream,  until  we  reach  the  solid  ledge  from  which  they 
were  derived,  showing  the  same  gradation  as  the  beach 
pebbles  when  followed  back  to  the  parent  cliff. 

The  pebbles,  however,  not  only  grind  each  other,  but 
also  the  solid  rocks  which  form  the  bed  of  the  streams 
in  many  places,  and  these  are  gradually  worn  away. 
When  the  rocky  bed  is  uneven  and  the  water  is  swift, 
pebbles  collect  in  hollows  where  eddies  are  formed,  by 
which  they  are  kept  whirling  and  turning,  and  the  hollow 
is  deepened  to  a  pot-hole,  while  the  pebbles,  the  river's 
tools,  are  worn  out  at  the  same  time. 

By  these  observations  we  learn  not  only  that  run- 
ning water  carries  away  sand  and  clay  already  formed, 
but  that  it  also  has  great  power  of  grinding  down  hard 


l8  DYNAMICAL   GEOLOGY. 

rocks  to  sand  and  clay.J  Of  course  the  pulverized  rock 
always  moves  in  the  same  direction  as  the  stream  which 
carries  it ;  and,  in  a  certain  sense,  all  streams  run  in 
one  direction,  viz.,  toward  the  sea.  Therefore  the  con- 
stant tendency  of  the  rain  falling  upon  the  land  is  to 
break  up  the  rocks  by  chemical  and  mechanical  action 
and  transport  the  debris  to  the  sea. 

Rivers,  as  we  all  know,  are  continually  uniting  to 
form  larger  and  larger  streams ;  and  thus  the  drainage 
of  a  wide  area  sometimes,  as  in  the  case  of  the  Missis- 
sippi Valley,  reaches  the  sea  through  a  single  mouth. 
By  careful  measurements  made  at  the  mouth  of  the  Mis- 
sissippi it  has  been  shown  that  the  20,000,000,000,000 
cubic  feet  of  water  discharged  into  the  Gulf  of  Mexico 
annually  carries  with  it  no  less  than  7,500,000,000 
cubic  feet  of  sand,  clay,  and  dissolved  mineral  matter ; 
and  this,  spread  over  the  whole  Mississippi  basin,  would 
form  a  layer  a  little  more  than  -^Vs  °f  a  f°ot  m  thick- 
ness. So  that  we  may  conclude  that  the  surface  of  the 
continent  is  being  cut  down  on  the  average  about  one 
foot  in  five  thousand  years. 

We  can  only  allude  in  passing  to  the  very  important 
geological  action  of  water  in  the  solid  state,  as  in  gla- 
ciers and  icebergs.  The  moisture  precipitated  from 
the  atmosphere,  and  falling  as  rain,  makes  ordinary 
rivers ;  but  falling  in  the  form  of  snow  in  cold  regions, 
where  more  snow  falls  than  is  melted,  the  excess  ac- 
cumulates and  is  gradually  compacted  to  ice,  which, 
like  water,  yields  to  the  enormous  pressure  of  its  own 
mass  and  flows  toward  lower  levels.  When  the  ice- 
river  reaches  the  sea  it  breaks  off  in  huge  blocks,  which 
float  away  as  icebergs.  Moving  ice,  like  moving  water, 


AQUEOUS   AGENCIES.  19 

is  a  powerful  agent  of  erosion ;  and  the  glacial  marks 
or  scratches  observable  upon  the  ledges  everywhere 
in  the  Northern  States  and  Canada  attest  the  magni- 
tude of  the  ice-action  at  a  comparatively  recent  period. 

We  have  already  noticed  incidentally  the  powerful 
disintegrating  action  of  water  where  it  freezes  in  the 
joints  and  pores  of  the  rocks ;  and  it  is  probable  that 
it  thus  facilitates  the  destruction  of  the  rocks  in  cold 
countries  nearly  as  much  as  the  higher  temperature 
and  greater  rain-fall  do  in  warm  countries. 

Our  observations  up  to  this  point  show  us  that  ero- 
sion, by  which  we  mean  the  breaking  up  by  chemical 
and  mechanical  action  of  the  rocks  of  the  land  and  the 
transportation  of  the  debris  into  the  sea,  is  one  great 
result  accomplished  by  the  inorganic  aqueous  agencies. 

MECHANICAL  DEPOSITION.  —  Next  let  us  notice  what 
becomes  of  all  this  vast  amount  of  clay,  sand,  and  gravel 
after  it  is  washed  into  the  ocean.  By  taking  up  a  glass 
of  turbid  water  from  our  roadside  rill,  and  observing 
that  as  soon  as  the  water  is  undisturbed  the  sand  and 
clay  begin  to  settle,  we  learn  that  the  solid  matter  is 
held  in  suspension  by  the  motion  of  the  water.  But 
it  does  not  remain  in  suspension  long  after  being  washed 
into  the  sea,  for  otherwise  the  sea  would,  in  the  course 
of  time,  become  turbid  for  long  distances  from  shore  ; 
and  it  is  a  well-known  fact  that  the  sea-water  is  usually 
clear  and  free  from  sensible  turbidity  close  along  shore 
and  even  near  the  mouths  of  large  rivers,  while  at  a 
distance  of  only  50  or  100  miles  we  find  the  transpar- 
ency of  the  central  ocean. 

Putting  these  facts  together,  we  see  that  the  ocean, 
noth withstanding  tl?e  ceaseless  and  often  violent  undu- 


*0  DYNAMICAL    GEOLOGY. 

lations  of  its  surface,  must  be  as  a  whole  a  vast  body 
of  still  water ;  and  to  the  reflecting  mind  the  almost 
perfect  tranquillity  of  the  ocean  is  one  of  its  most  im- 
pressive features.  For  it  is  in  striking  contrast,  in  this 
respect,  with  the  more  mobile  aerial  ocean  above  it. 

We  have  got  hold,  now,  of  two  facts  of  great  geo- 
logical importance  :  ( i )  The  debris  washed  off  the  land 
by  waves  and  rivers  into  the  still  water  of  the  ocean 
very  soon  settles  to  the  bottom ;  and  (2)  it  nearly  all 
settles  on  that  part  of  the  ocean-floor  near  the  land. 

And  now  we  have  in  view  the  second  great  office 
of  the  inorganic  aqueous  agencies,  —  deposition,  the 
counterpart  or  complement  of  erosion. 

The  land  is  the  great  theatre  of  erosion  and  the  sea 
of  deposition  ;  the  rocks  which  are  constantly  wasting 
away  on  the  former  are  as  constantly  renewed  in  the 
latter. 

We  will  now  observe  the  process  of  deposition  a  little 
more  closely.  Each  of  these  two  bottles  contains  the 
same  amount  of  fine  yellow  clay,  but  in  one  the  water 
is  fresh,  and  in  the  other  it  is  salt.  At  the  beginning 
of  the  lesson,  as  you  may  have  observed,  I  brought  the 
clay  in  both  bottles  into  suspension  by  violent  agita- 
tion, and  since  then  they  have  remained  undisturbed. 
The  main  point  is  that  the  salt  water  has  become  quite 
clear,  while  the  fresh  water  is  still  distinctly  turbid, 
showing  that  the  salt  favors  the  rapid  deposition  of  the 
clay.  At  the  second  lecture,  a  week  later,  these  two 
bottles,  yet  undisturbed,  were  exhibited,  and  the  fresh 
water  seen  to  be  still  sensibly  turbid.  The  fact  is,  the 
clay  is  not  held  in  suspension  wholly  by  the  motion  of 
the  water ;  but,  just  as  in  the  case  of  dust  in  the  atmos- 


AQUEOUS  AGENCIES.  21 

phere,  a  small  portion  of  the  medium  is  condensed 
around  or  adheres  to  each  solid  particle,  *>.,  each 
clay  particle  in  our  experiment  has  an  atmosphere  of 
water  which  moves  with  it  and  buoys  it  up.  Now  the 
effect  of  the  salt  is  to  diminish  the  adhesion  of  the 
water  to  the  particles,  i.e.,  to  diminish  their  atmos- 
pheres, and  consequently  their  buoyancy.  The  dimin- 
ished adhesion  of  the  salt  water  is  well  shown  by  the 
smaller  drops  which  it  forms  on  a  glass  rod. 

The  geological  importance  of  this  principle  is  very 
great ;  for  it  is  undoubtedly  largely  to  the  saltness  of 
the  sea  that  we  owe  its  transparency,  and  the  fact  that 
the  fine,  clayey  sediment  from  the  land,  like  the  coarse, 
is  deposited  near  the  shore. 

This  bottle  of  fresh  water  contains  some  fine  gravel, 
coarse  sand,  fine  sand,  and  clay.  By  agitating  the 
water,  all  this  material  is  brought  into  suspension. 
Now,  suddenly  placing  the  bottle  in  a  state  of  rest,  we 
observe  that  the  gravel  falls  to  the  bottom  almost  instant- 
ly, followed  quickly  by  the  coarse  sand,  and  very  soon 
afterward  by  the  fine  sand  ;  and  then  there  appears  to 
be  a  pause,  the  fine  particles  of  clay  all  remain  in  sus- 
pension ;  but  finally,  when  the  water  is  quite  motion- 
less, they  begin  to  settle  ;  they  fall  very  slowly,  however, 
and  the  water  will  not  be  clear  for  hours. 

This  is  a  very  instructive  experiment.  We  learn- 
from  it : 

.  First,  that  the  power  of  the  water  to  hold  particles 
in  suspension  is  inversely  proportional  to  the  size  of 
the  particles ; 

Second,  that  all  materials  deposited  in  water  are  as* 
sorted  according  to  size  ; 


22  DYNAMICAL    GEOLOGY. 

Third,  and  this  is  one  of  the  most  important  facts  in 
geology,  all  water-deposited  sediments  are  arranged  in 
horizontal  layers,  i.e.,  are  stratified.  And  we  have  now 
traced  to  its  conclusion,  though  very  briefly,  the  process 
of  the  formation  of  one  great  division  of  stratified  rocks, 
—  the  mechanically-formed  orfragmental  rocks.  These 
are  so  called  because  the  clay,  sand,  and  gravel  are, 
in  every  instance,  fragments  of  pre-existing  rocks ;  and 
because  the  formation,  transportation,  and  especially 
the  deposition  of  these  fragments,  are  the  work  chiefly 
or  entirely  of  mechanical  forces. 

CHEMICAL  DEPOSITION.  —  It  is  a  well-known  fact  that 
the  sea  holds  in  solution  vast  amounts  of  common  salt 
as  well  as  many  other  substances;  and  analyses  of 
river-waters  show  that  dissolved  minerals  derived  from 
the  chemical  decomposition  of  the  rocks  of  the  land  are 
being  constantly  carried  into  the  sea. 

Portions  of  the  sea  which  are  cut  off  from  the  main 
body,  and  which  are  gradually  drying  up,  like  the 
Great  Salt  Lake,  Dead  Sea,  and  Caspian  Sea,  become 
saturated  solutions  of  the  various  dissolved  minerals, 
and  these  are  slowly  deposited.  This  process  is  very 
nicely  illustrated  along  our  shores  in  summer,  where> 
during  storms,  salt-water  spray  is  thrown  above  the 
reach  of  the  tides,  and,  collecting  in  hollows  in  the 
rocks,  gradually  dries  up,  leaving  behind  a  crust  of  salt. 

When  the  sea  lays  down  matter  which  it  held  in  sus- 
pension, we  call  the  process  mechanical  deposition,  and 
the  result  is  mecham'ca/fy-formed  rocks. 

But  when  it  lays  down  matter  which  it  held  in  solution, 
we  call  the  process  chemical  deposition,  and  the  result 
is  chemicall-iQimzd.  rocks. 


AQUEOUS  AGENCIES.  23 

The  principal  substances  which  the  sea  deposits 
chemically  are  common  salt,  forming  beds  of  rock-salt ; 
sulphate  of  calcium,  forming  beds  of  gypsum ;  carbonate 
of  calcium,  forming  beds  of  limestone  ;  and  the  double 
carbonate  of  calcium  and  magnesium,  forming  beds  of 
dolomite. 

Inorganic  deposition,  like  inorganic  erosion,  is  both 
chemical  and  mechanical. 

2.  Animals  and  Plants,  or  Organic  Agencies. 

We  turn  now  to  the  consideration  of  the  organic 
agencies.  And  I  will  merely  allude  in  passing  to  the 
vast  importance  of  the  fossil  organic  remains  found 
in  the  stratified  rocks  as  marks  by  which  to  determine 
the  relative  ages  of  the  formations. 

As  regards  the  destruction  of  rocks — erosion — plants 
and  animals  are  almost  powerless ;  but  in  the  role  of 
rock-makers  they  play  a  very  important  part,  being  very 
efficient  agents  of  deposition. 

FORMATION  OF  COALS  AND  BITUMENS.  —  Specimen 
No.  8  is  an  example  of  peat  from  the  vicinity  of  Bos- 
ton ;  but  just  as  good  specimens  may  be  obtained  in 
thousands  of  places  in  this  and  other  States. 

The  general  physical  conditions  under  which  peat  is 
formed  are  familiar  facts.  We  require  simply  low,  level 
land,  covered  with  a  thin  sheet  of  water  and  abundant 
vegetation;  in  other  words,  a  marsh  or  swamp.  If 
plants  decay  on  the  dry  land,  the  decomposition  is 
complete ;  they  are  burned  up  by  the  oxygen  of  the 
air  to  carbon  dioxide  and  water  just  as  surely  as  if 


24  DYNAMICAL    GEOLOGY. 

they  had  been  thrown  into  a  furnace,  though  less 
rapidly,  and  nothing  is  returned  to  the  soil  but  what 
had  been  taken  from  it  by  the  plants  during  their 
growth.  But  if  the  plants  decay  under  water,  as  in 
a  peat-marsh  or  bog,  the  decay  is  incomplete,  and 
most  of  the  carbon  of  the  wood  is  left  behind.  Now, 
if  this  incomplete  combustion  of  vegetable  tissues  takes 
place  in  a  charcoal-pit,  where  the  wood  is  out  of  con- 
tact with  air  from  being  covered  with  earth,  we  call  the 
carbonaceous  product  charcoal ;  but  if  under  the  water 
of  a  marsh,  in  Nature's  laboratory,  we  call  the  product 
peat.  Peat  is  simply  a  natural  charcoal ;  and,  just  as  in 
ordinary  charcoal,  its  vegetable  origin  is  always  perfectly 
evident.  But  when  the  deposit  becomes  thicker,  and 
especially  when  it  is  buried  under  thick  formations 
of  other  rocks,  like  sand  and  clay,  the  great  pressure 
consolidates  the  peat ;  it  becomes  gradually  more  min- 
eralized and  shining,  shows  the  vegetable  tissues  less  dis- 
tinctly, becomes  more  nearly  pure  carbon,  and  we  call 
it  in  succession  lignite,  bituminous  coal,  and  anthracite. 
This  is,  briefly,  the  way  in  which  all  varieties  of  coal, 
as  well  as  the  more  solid  kinds  of  bitumen,  like  asphal- 
tum,  are  formed.  But  the  lighter  forms  of  bitumen, 
such  as  petroleum  and  naphtha,  are  derived  mainly, 
if  not  entirely,  from  the  partial  decomposition  of  animal 
tissues.  These,  ic  is  well  known,  decay  much  more 
readily  than  vegetable  tissues  ;  and  the  water  of  an  or- 
dinary marsh  or  lake  contains  sufficient  oxygen  for 
their  complete  and  rapid  decomposition.  In  the 
deeper  parts  of  the  ocean,  however,  the  conditions  are 
very  different,  for  recent  researches  have  shown,  con- 
trary to  the  old  idea,  that  the  deep  sea  holds  an  abun- 


AQUEOUS  AGENCIES.  25 

dant  fauna.  All  grades  of  animal  life,  from  the  highest 
to  the  lowest,  have  need  of  a  constant  supply  of  oxy- 
gen. On  the  land  vegetation  is  constantly  returning  to 
the  air  the  oxygen  consumed  by  animals,  but  in  the 
abysses  of  the  ocean  vegetable  life  is  scarce  or  wanting  ; 
and  hence  it  must  result  that  over  these  greater  than 
continental  areas  countless  myriads  of  animals  are  living 
habitually  on  short  rations  of  oxygen,  and  in  water  well 
charged  with  carbon  dioxide,  the  product  of  animal 
respiration.  As  a  consequence,  when  these  animals  die 
their  tissues  do  not  find  the  oxygen  essential  for  their 
perfect  decomposition,  and  in  the  course  of  time 
become  buried,  in  a  half-decayed  state,  in  the  ever- 
increasing  sediments  of  the  ocean-floor. 

It  is  important  to  observe  that  an  abundance  of  or- 
ganic matter  decaying  under  water  is  not  the  only  con- 
dition essential  to  the  formation  of  beds  of  coal  and 
bitumen ;  for  this  condition  is  realized  in  the  luxuriant 
growth  of  sea-weeds  fringing  the  coast  in  every  quarter 
of  the  globe  ;  and  yet  coals  and  bitumens  are  rarely  of 
sea-shore  origin.  These  organic  products,  even  under 
the  most  favorable  circumstances,  accumulate  with 
extreme  slowness  ;  far  more  slowly,  as  a  rule,  than  the 
ordinary  mechanical  sediments,  like  sand  and  clay, 
with  which  they  are  mixed,  and  in  which  they  are  often 
completely  lost.  Consequently,  although  the  deposi- 
tion of  the  carbonized  remains  of  plants  and  animals  is 
taking  place  in  nearly  all  seas,  lakes,  and  marshes,  it  is 
only  in  those  places  where  there  is  little  or  no  mechani- 
cal sediment  that  they  can  predominate  so  as  to  build 
up  beds  pure  enough  to  be  called  coal  or  bitumen.  In 
all  other  cases  we  get  merely  more  or  less  carbonaceous 


26  DYNAMICAL    GEOLOGY. 

sand  or  clay.  Now  these  especially  favorable  locali- 
ties will  manifestly  not  be  often  found  along  the  sea- 
shore, where  we  have  strewn  the  sand  and  clay  brought 
down  by  rivers  or  washed  off  the  land  directly  by 
the  ever-active  surf;  but  they  must  exist  in  the  central 
portions  of  the  ocean,  where  there  is  almost  no  mechan- 
ical sediment  and  yet  an  abundance  of  life,  and  in 
swamps  and  marshes,  where  there  is  scarcely  sufficient 
water  to  cover  the  vegetation,  and  no  waves  or  currents 
to  wash  down  the  soil  from  the  surrounding  hills. 

FORMATION  OF  IRON-ORES.  —  The  iron-ores  are 
another  class  of  rocks  which  are  formed  only  through 
the  agency  of  organic  matter.  Iron  is  an  abundant 
and  wide-spread  element  in  the  earth's  crust,  and,  but 
for  the  intervention  of  life,  we  might  say  that,  while 
there  is  iron  everywhere,  there  is  not  much  of  it  in 
any  one  place,  since  it  is  originally  very  thinly  diffused. 
All  rocks  and  soils  contain  iron,  but  it  is  mainly  in  the 
form  of  the  peroxide,  in  which  state  it  is  entirely  insol- 
uble, and  hence  cannot  be  soaked  out  of  the  soil  by 
the  rain-water  and  concentrated  by  the  evaporation  of 
the  water  at  lower  levels  in  ponds  and  marshes,  as  a 
soluble  substance  like  salt  would  be.  If  carried  off 
with  the  sand  and  clay,  by  the  mechanical  action  of 
water,  it  remains  uniformly  mixed  with  them,  and  there 
is  no  tendency  to  its  separation  and  concentration  so 
as  to  form  a  true  iron-ore. 

But  what  water  cannot  do  alone  is  accomplished 
very  readily  when  the  water  is  aided  by  decaying  organic 
matter,  which  is  always  hungry  for  oxygen,  being,  in 
the  language  of  the  chemist,  a  powerful  reducing  agent. 
The  soil,  in  most  places,  has  a  superficial  stratum  of 


AQUEOUS  AGENCIES.  27 

vegetable  mould  or  half-decayed  vegetation.  The  rain- 
water percolates  through  this  and  dissolves  more  or 
less  of  the  organic  matter,  which  is  thus  carried  down 
into  the  sand  and  clay  beneath  and  brought  in  contact 
with  the  ferric  oxide,  from  which  it  takes  a  certain 
proportion  of  oxygen,  reducing  the  ferric  to  the  ferrous 
oxide.  At  the  same  time  the  vegetation  is  burned  up 
by  the  oxygen  thus  obtained,  forming  carbon  dioxide, 
which  immediately  combines  with  the  ferrous  oxide, 
forming  carbonate  of  iron,  which,  being  soluble  under 
these  conditions,  is  carried  along  by  the  water  as  it 
gradually  finds  its  way  by  subterranean  drainage  to  the 
bottom  of  the  valley  and  emerges  in  a  swamp  or  marsh. 
Here  one  of  two  things  will  happen  :  If  the  marsh 
contains  little  or  no  decaying  vegetation,  then  as  soon 
as  the  ferrous  carbonate  brought  down  from  the  hills 
is  exposed  to  the  air  it  is  decomposed,  the  carbon 
dioxide  escapes,  and  the  iron,  taking  on  oxygen  from 
the  air,  returns  to  its  original  ferric  condition ;  and 
being  then  quite  insoluble,  it  is  deposited  as  a  loose, 
porous,  earthy  mass,  commonly  known  as  bog-iron- 
ore,  which  becomes  gradually  more  solid  and  finally 
even  crystalline  through  the  subsequent  action  of 
heat  and  pressure.  When  first  deposited,  the  ferric 
oxide  is  combined  with  water  or  hydrated,  and  is 
then  known  as  limonite  (specimen  No.  1 2)  ;  at  a 
later  period  the  water  is  expelled,  and  we  call  the  ore 
hematite  (specimen  No.  13)  ;  and  at  a  still  later  age 
it  loses  part  of  its  oxygen,  becomes  magnetic  and  more 
crystalline,  and  is  then  known  as  magnetite  (specimen 
No.  14).  Thus  it  is  seen  that  the  iron-ores,  as  we 
pass  from  bog-limonite  to  magnetite,  form  a  natural 


28  DYNAMICAL    GEOLOGY. 

series  similar  to  and  parallel  with  that  afforded  by  the 
coals  as  we  pass  from  peat  to  graphite. 

If  the  drainage  from  the  hills  is  into  a  marsh  contain- 
ing an  abundance  of  decaying  vegetation,  i.e.,  if  peat 
is  forming  there,  the  ferrous  carbonate,  in  the  presence 
of  the  more  greedy  organic  matter,  will  be  unable  to 
obtain  oxygen  from  the  air  ;  and  as  the  evaporation  of 
the  water  goes  on,  it  will  sooner  or  later  become  sat- 
urated with  this  salt,  and  the  latter  will  be  deposited. 
Here  we  find  an  explanation  of  a  fact  often  observed 
by  geologists,  viz.,  that  the  carbonate  iron-ores  are 
usually  associated  with  beds  of  coal, 

The  formation  of  the  iron-ores,  like  that  of  the  coals 
and  bitumens,  is  a  slow  process ;  and  the  ores,  like  the 
coals,  etc.,  will  be  pure  only  where  there  is  a  complete 
absence  of  mechanical  sediment,  a  condition  that  is 
realized  most  nearly  in  marshes. 

FORMATION  OF  LIMESTONE,  DIATOMACEOUS  EARTH,  ETC. 
—  Marine  animals  take  from  the  sea- water  certain  min- 
eral substances,  especially  silica  and  carbonate  of 
calcium,  to  form  their  skeletons.  Silica  is  used  only 
by  the  lowest  organisms,  such  as  Radiolaria,  Sponges, 
and  the  minute  unicellular  plants,  Diatoms.  The  prin- 
cipal animals  secreting  carbonate  of  calcium  are  Corals 
and  Mollusks.  These  hard  parts  of  the  organisms 
remain  undissolved  after  death ;  and  over  portions  of 
the  ocean-floor  where  there  is  but  little  of  other  kinds 
of  sediment  they  form  the  main  part  of  the  deposits, 
and  in  the  course  of  ages  build  up  very  extensive  for- 
mations which  we  call  diatomaceous  earth  or  tripolite, 
if  the  organisms  are  siliceous,  or  limestone  if  they  are 
calcareous.  A  very  satisfactory  account  of  the  for- 


AQUEOUS  AGENCIES.  29 

mation  of  limestone  on  a  stupendous  scale  by  the 
polyps  in  coral  reefs  and  islands  is  contained  in  No. 
IV.  of  this  series  of  guides. 

The  rocks  here  considered  may  be,  and,  as  we  have 
already  seen,  sometimes  are,  deposited  in  a  purely 
chemical  way,  without  the  aid  of  life ;  and  it  is  impor- 
tant to  observe  that  in  no  case  do  the  organisms  make 
the  silica  and  carbonate  of  calcium  of  their  skeletons, 
but  they  simply  appropriate  and  reduce  to  the  solid  state 
what  exists  ready  made  in  solution  in  the  sea-water. 
These  minerals,  and  others,  as  we  know,  are  produced 
by  the  decomposition  of  the  rocks  of  the  land,  and  are 
being  constantly  carried  into  the  sea  by  rivers ;  and, 
if  there  were  no  animals  in  the  sea,  these  processes 
would  still  go  on  until  the  sea-water  became  saturated 
with  these  substances,  when  their  precipitation  as 
limestone,  etc.,  would  necessarily  follow.  Hence  it  is 
clear  that  all  the  animals  do  is  to  effect  the  precipitation 
of  certain  minerals  somewhat  sooner  than  it  would 
otherwise  occur ;  so  that  from  a  geological  standpoint 
the  differences  between  chemical  and  organic  deposition 
are  not  great. 

This  section  of  our  subject  may  be  summarized  as 
follows  :  Animals  and  plants  contribute  to  the  forma- 
tion of  rocks  in  three  distinct  ways  :  — 

i.  During  their  growth  they  deoxidize  carbon  dioxide 
and  water,  and  reduce  to  the  solid  state  in  their  tissues 
carbon  and  the  permanent  gases  oxygen,  hydrogen,  and 
nitrogen ;  and  after  death,  through  the  accumulation 
of  the  half-decayed  tissues  in  favorable  localities,  — 
marshes,  etc.,  —  these  elements  are  added  to  the  solid 
crust  of  the  earth  in  the  form  of  coal  and  bitumen. 


3o  DYNAMICAL    GEOLOGY. 

2.  During  the  decomposition,  i.e.,  oxidation,  of  the 
organic  tissues,  the  iron  existing  everywhere  in  the  soil 
is  partially  deoxidized,  and,  being  thus  rendered  soluble, 
is  removed  by  rain-water  and  concentrated  in  low  places, 
forming  beds  of  iron-ore. 

3.  Through  the  agency  of  marine  organisms,  certain 
mineral  substances  are  being  constantly  removed  from 
the  sea-water  and   deposited   upon   the   ocean   floor, 
forming  various  calcareous  and  siliceous  rocks. 

I  now  bring  our  study  of  the  aqueous  or  superficial 
agencies  to  a  conclusion  by  noting  once  more  that  the 
great  geological  results  accomplished  by  air,  water,  and 
organic  matter  or  life  are  :  ( i )  Erosion,  or  the  wearing 
away  of  the  surface  of  the  land;  and  (2)  Deposition, 
or  the  formation  from  the  debris  of  the  eroded  land  of 
two  great  classes  of  stratified  rocks,  —  the  mechan- 
ically formed  or  fragmental  rocks,  and  the  chemically 
and  organically  formed  rocks. 


II.    IGNEOUS  AGENCIES. 

We  pass  next  to  a  very  brief  consideration  of  opera- 
tions that  originate  below  the  earth's  surface.  The 
records  of  deep  mines  and  artesian  wells  show  that  the 
temperature  of  the  ground  always  increases  downwards 
from  the  surface  ;  and  the  much  higher  temperatures  of 
hot  springs  and  volcanoes  show  that  the  heat  continues 
to  increase  to  a  great  depth,  and  is  not  a  merely  super- 
ficial phenomenon.  The  observed  rate  of  increase  is 
not  uniform,  but  it  seldom  varies  far  from  the  average, 
which  is  about  i°  Fahr.  per  53  feet  of  vertical  descent, 
or,  in  round  numbers,  100°  per  mile.  This  rate,  if 


IGNEOUS  AGENCIES.  31 

continued,  would  give  a  very  high  temperature  at  points 
only  a  few  miles  below  the  surface  ;  and  until  within  a 
few  years  the  idea  was  generally  accepted  by  geologists 
that  the  increase  of  temperature  is  sensibly  uniform 
for  an  indefinite  distance  downward ;  that  in  the  cen- 
tral regions  of  the  earth  the  temperature  is  far  higher 
than  anything  we  can  conceive,  and  that  everywhere 
below  a  depth  of  20  to  40  miles  the  temperature  is 
above  the  fusing-point  of  all  rocks ;  and  hence  that 
the  earth  is  an  incandescent  liquid  globe  covered  by 
a  thin  shell  or  crust  of  cold,  solid  rock. 

Our  limited  space  will  not  permit  us  to  enter  into  a 
discussion  of  the  condition  of  the  earth's  interior,  and 
I  will  merely  point  out  in  a  few  sentences  the  posi- 
tion occupied  by  geologists  at  the  present  time.  The 
reasoning  of  Thompson  has  shown  that  the  tempera- 
ture cannot  increase  downward  at  a  uniform  rate,  but 
at  a  constantly  and  rapidly  diminishing  rate ;  and 
that  everywhere  below  a  depth  of  300  miles  the  tem- 
perature is  probably  sensibly  the  same,  and  nowhere, 
probably,  above  8000°  to  10,000°  Fahr. 

Unlike  water,  all  rocks  contract  on  solidifying  and 
expand  on  melting,  and  consequently  the  high  pres- 
sures to  which  they  are  subjected  in  the  earth's  inte- 
rior— 10,000,000  to  20,000,000  pounds  per  square 
inch  —  must  raise  their  fusing-points  enormously,  and 
the  probabilities  are  that  they  are  solid,  in  spite  of  the 
high  temperature.  But  Thompson  and  Darwin  have 
shown  us  farther  that  the  phenomena  of  the  oceanic 
tides  could  not  be  what  they  are  known  to  be  if  the 
earth  were  any  less  rigid  than  a  globe  of  solid  steel ; 
while  Hopkins  has  proved  that  the  astronomical  phe- 


32  DYNAMICAL    GEOLOGY. 

nomena  of  precession  and  nutation  could  not  be  what 
they  are  if  the  earth's  crust  were  less  than  800  or  1000 
miles  thick.  Putting  these  considerations  together, 
geologists  are  almost  universally  agreed  that,  while  the 
earth  has  an  incandescent  interior,  it  is  still  continu- 
ously solid  from  centre  to  circumference,  with  the 
exception  of  a  thin  plastic  stratum  at  a  depth  not 
exceeding  40  or  50  miles,  which  forms  the  seat  of  vol- 
canic action. 

The  earth  is  not  only  a  very  hot  body,  but  it  is  rotat- 
ing through  almost  absolutely  cold  space,  and  there- 
fore must  be  a  cooling  body.  But,  except  at  the  very 
beginning  of  the  cooling,  the  loss  of  heat  has  gone  on 
almost  entirely  from  the  interior;  and  since  cooling 
means  contraction,  the  heated  interior  must  be  con- 
stantly tending  to  shrink  away  from  the  cold  external 
crust. 

Of  course  no  actual  separation  between  the  crust 
and  interior  or  nucleus  can  take  place,  but  there  is  no 
doubt  that  the  crust  is  left  unsupported  to  a  certain 
extent,  and  it  must  then  behave  like  an  arch  with  a 
radius  of  4000  miles,  and  the  result  is  an  enormous 
horizontal  or  tangential  pressure. 

This  lateral  pressure  in  the  earth's  crust  is  one  of  the 
most  important  and  most  generally  accepted  facts  in 
geology,  and  lies  at  the  bottom  of  many  geological  the- 
ories. According  to  what  seems  to  me  to  be  the  most 
probable  theory  of  the  origin  of  continents  and  ocean- 
basins,  they  are  broad  upward  and  downward  bendings 
or  arches  into  which  the  crust  is  thrown  by  the  tan- 
gential pressure.  Finally,  the  strain  becomes  great 
enough  to  crush  the  crust  along  those  lines  where  it  is 


IGNEOUS  AGENCIES.  33 

weakest.  When  the  crust  is  thus  mashed  up  by  hori- 
zontal pressure,  a  mountain  range  is  formed,  the  crust 
becomes  enormously  thicker,  and  a  weak  place  becomes 
a  strong  one. 

During  the  formation  of  mountains  the  stratified 
rocks,  which  were  originally  horizontal,  are  thrown  into 
folds  or  arches,  and  tipped  up  at  all  possible  angles  ; 
they  are  fractured  and  faults  produced ;  and  by  the 
immense  pressure  the  structure  known  as  slaty  cleavage 
is  developed.  In  fact,  a  vast  amount  and  variety  of 
structures  are  produced  during  the  growth  of  a  moun- 
tain range. 

These  great  earth-movements  are  not  always  per- 
fectly smooth  and  steady,  but  they  are  accompanied  by 
slipping  or  crushing  now  and  then  ;  and,  as  a  result 
of  the  shock  thus  produced,  a  swift  vibratory  movement 
or  jar,  which  we  know  as  an  earthquake,  runs  through 
the  earth's  crust. 

Extensive  fissures  are  also  formed,  opening  down  to 
the  regions  where  the  rocks  are  liquid  or  plastic,  and 
through  these  the  melted  rocks  flow  up  to  or  toward 
the  surface.  That  portion  which  flows  out  on  the 
surface  builds  up  a  volcanic  cone,  while  that  which 
cools  and  solidifies  below  the  surface,  in  the  fissures, 
forms  dikes.  Thus  among  the  igneous  or  eruptive 
rocks  we  have  two  great  classes,  —  the  dike  rocks  and 
the  volcanic  rocks. 

It  is  important  to  observe  that  all  these  subterranean 
operations  —  the  formation  of  continents,  of  mountain- 
ranges  with  all  their  attendant  phenomena  of  folds, 
faults  and  cleavage,  and  every  form  and  phase  of  earth- 
quake and  volcanic  activity  —  depend  upon  or  originate 


34  DYNAMICAL    GEOLOGY. 

in  the  interior  heat  of  the  earth.  And  over  against  the 
superficial  or  aqueous  agencies,  originating  in  the  solar 
heat  and  producing  the  stratified  or  sedimentary  rocks, 
we  set  the  subterranean  or  igneous  agencies  originating 
in  the  central  heat,  and  producing  the  unstratified  or 
eruptive  rocks. 


STRUCTURAL    GEOLOGY. 


IN  geology,  just  as  in  biology,  there  are  two  ways  of 
studying  structure,  —  the  small  way  and  the  large  way. 
In  the  case  of  an  organism,  we  may  select  a  single  part 
or  organ,  and,  disregarding  its  external  form  and  rela- 
tions to  other  parts,  observe  its  composition  and  minute 
structure,  the  various  forms  and  arrangements  of  the 
cells,  etc.  This  is  histology,  and  it  is  the  complement 
of  that  larger  method  of  studying  structure  which  is 
ordinarily  understood  by  anatomy. 

The  divisions  of  structural  geology  corresponding  to 
histology  and  anatomy  are  lithology  and  petrology. 
Lithology  is  an  in-door  science  ;  we  use  the  microscope 
largely,  and  work  with  hand  specimens  or  thin  sections 
of  the  rocks,  observing  the  composition  and  those 
small  structural  features  which  go  under  the  general 
name  of  texture. 

In  petrology,  on  the  other  hand,  we  consider  the 
larger  kinds  of  rock-structure,  such  as  stratification, 
jointing,  folds,  faults,  cleavage,  etc. ;  and  it  is  essen- 
tially an  out-door  science,  since  to  study  it  to  the 
best  advantage  we  must  have,  not  hand  specimens, 
but  ledges,  cliffs,  rail  way- cuttings,  gorges,  and  moun- 
tains. 


36  STRUCTURAL    GEOLOGY. 

LITHOLOGY. 

A  rock  is  any  mineral,  or  mixture  of  minerals,  oc- 
curring in  masses  of  considerable  size.  This  distinction 
of  size  is  the  only  one  that  can  be  made  between  rocks 
and  minerals,  and  that  is  very  indefinite.  A  rock, 
whether  composed  of  one  mineral  or  several,  is  always 
an  aggregate  ;  and  therefore  no  single  crystal  or  min- 
eral-grain can  properly  be  called  a  rock. 

Before  proceeding  to  study  particularly  the  various 
kinds  of  rocks,  a  little  more  preliminary  work  should 
be  done.  As  already  intimated,  the  more  important 
characteristics  of  rocks  may  be  grouped  under  two 
general  heads,  —  composition  and  texture. 

Composition   of  Rocks. 

Rocks  are  properly  defined  as  large  masses  or  ag- 
gregates of  mineral  matter,  consisting  in  some  cases 
of  one  and  in  other  cases  of  several  mineral  species. 
Hence  it  is  clear  that  the  composition  of  rocks  is  of 
two  kinds  :  chemical  and  mineralogical ;  for  the  va- 
rious chemical  elements  are  first  combined  to  form 
minerals,  and  then  the  minerals  are  combined  to  form 
rocks. 

Of  course  those  minerals  and  elements  which  can  be 
described  as  principal  or  important  rock-constituents 
must  be  the  common  minerals  and  elements.  Now  it 
is  a  very  important  and  convenient  fact  that  although 
chemists  recognize  about  sixty-five  elementary  sub- 
stances, and  these  are  combined  to  form  nearly  one 
thousand  mineral  species,  yet  both  the  common  ele- 
ments and  the  common  minerals  are  few  in  number. 


LITHOLOGY.  37 

So  that,  although  it  is  very  desirable  and  even 
necessary  for  the  student  of  lithology  to  know  some- 
thing of  chemistry  and  mineralogy,  it  by  no  means  fol- 
lows that  he  or  she  must  be  master  of  those  sciences. 
A  knowledge  of  the  chemical  and  physical  character- 
istics of  a  few  common  minerals  is  all  that  is  absolutely 
essential,  though  it  may  be  added  that  an  excess  of 
wisdom  in  these  directions  is  no  disadvantage. 

Chemical  Composition  of  Rocks. 

The  elementary  substances  of  which  rocks  are  chiefly 
composed,  which  make  up  the  main  mass  of  the  earth 
so  far  as  we  are  acquainted  with  it,  number  only 
fourteen :  — 

Non-Metallic  or  Acidic  Elements.  —  Oxygen,  silicon, 
carbon,  sulphur,  chlorine,  phosphorus,  and  fluorine. 

Metallic  or  Basic  Elements.  —  Aluminum,  magne- 
sium, calcium,  iron,  sodium,  potassium,  and  hydrogen. 

The  elements  are  named  in  each  group  in  about  the 
order  of  their  relative  abundance ;  and  to  give  some 
idea  of  the  enormous  differences  in  this  respect  it  may 
be  stated  that  two  of  the  elements  —  oxygen  and  sili- 
con —  form  more  than  half  of  the  earth's  crust. 

Silicon,  calcium,  and  fluorine,  although  exceedingly 
abundant,  are  also  very  difficult  to  obtain  in  the  free  or 
uncombined  state,  and  specimens  large  enough  to  ex- 
hibit to  a  class  would  be  very  expensive.  With  these 
exceptions,  however,  examples  of  these  common  rock- 
forming  elements  are  easily  obtained. 

My  purpose  in  calling  attention  to  this  point  is  sim- 
ply to  suggest  that  the  proper  way  to  begin  the  study 
ef  minerals  and  rocks  with  children  is  to  first  familiarize 


38  STRUCTURAL  GEOLOGY. 

them  with  the  elements  of  which  they  are  composed. 
The  most  important  thing  to  be  known  about  any  min- 
eral is  its  chemical  composition ;  and  when  a  child  is 
told  that  a  mineral  —  corundum,  for  example  —  is  com- 
posed of  oxygen  and  aluminum,  he  should  have  a  dis- 
tinct conception  of  the  properties  of  each  of  those 
elements,  for  otherwise  corundum  is  for  him  a  mere 
compound  of  names. 

It  is  very  important,  too,  if  the  pupil  has  not  already 
studied  chemistry,  that  he  should  be  led  to  some  com- 
prehension of  the  nature  of  chemical  union  and  of  the 
difference  between  a  chemical  compound  and  a  me- 
chanical mixture.  For  this  purpose  a  few  simple  ex- 
periments (the  details  of  which  would  be  out  of  place 
here)  with  the  more  common  and  familiar  elements 
will  be  sufficient.  Mrs.  Richard's  "  First  Lessons  in 
Minerals  "  should  be  introduced  here. 

Mineralogical  Composition  of  Rocks. 

The  fourteen  elements  named  above  are  combined  to 
form  about  fifty  minerals  with  which  the  student  of 
geology  should  be  acquainted  ;  but  not  more  than  one- 
half  of  these  are  of  the  first  importance.  It  is  desired 
to  lay  especial  emphasis  upon  the  importance  of  a  per- 
fect familiarity  with  these  few  common  minerals. 
There  is  nothing  else  in  the  whole  range  of  geology  so 
easily  acquired  which  is  at  the  same  time  so  valuable  ; 
for  it  is  entirely  impossible  to  comprehend  the  defini- 
tions of  rocks,  or  to  recognize  rocks  certainly  and  sci- 
entifically, unless  we  are  acquainted  with  their  con- 
stituent minerals. 

With  one  or  two  exceptions,  these  common  rock- 
forming  minerals  may  be  easily  distinguished  by  their 


LIT  HO  LOGY.  39 

physical  characters  alone,  so  that  their  certain  recogni- 
tion is  a  matter  of  the  simplest  observation,  and  entirely 
within  the  capacity  of  young  children.  Furthermore, 
being  common,  specimens  of  these  minerals  are  very 
easily  obtained,  so  that  there  is  no  reason  why  teachers 
should  not  here  adopt  the  best  method  and  place  a 
specimen  of  each  mineral  in  the  hands  of  each  pupil. 
Typical  examples,  large  enough  to  show  the  character- 
istics well,  ought  not  to  cost,  on  the  average,  over  two 
cents  apiece. 

A  MINERAL  is  an  inorganic  body  having  theoretically 
a  definite  chemical  composition,  and  usually  a  regular 
geometric  form. 

THE  PRINCIPAL  CHARACTERISTICS  OF  MINERALS.  — 
These  may  be  grouped  under  the  following  general 
heads : — 

(i)  Composition,  (2)  Crystalline  form,  (3)  Hardness, 
(4)  Specific  gravity,  (5)  Lustre,  (6)  Color  and  Streak. 

i.  Composition.  —  This,  according  to  the  definition 
of  a  mineral,  ought  to  be  definite,  and  expressible  by  a 
chemical  formula.  When  it  is  not  so,  we  usually  con- 
sider that  the  mineral  is  partially  decomposed,  or  that 
we  are  dealing  with  a  mixture  of  minerals.  It  is  well 
to  impress  upon  the  mind  of  the  pupil  the  important 
fact  that  the  more  fundamental  properties  of  the  ele- 
ments, such  as  specific  gravity  and  lustre,  are  not  lost 
when  they  combine,  but  may  be  traced  in  the  com- 
pounds. In  other  words,  the  properties  of  minerals 
are,  in  a  very  large  degree,  the  average  of  the  proper- 
ties of  the  elements  of  which  they  are  composed  ;  min- 
erals in  which  heavy  metallic  elements  predominate 
being  heavy  and  metallic,  and  vice  versa. 


40  STRUCTURAL    GEOLOGY. 

To  fully  appreciate  this  point  it  is  only  necessary 
to  compare  a  mineral  like  galenite  —  a  common  ore 
of  lead,  and  containing  nearly  87  per  cent,  of  that 
heavy  metal;  or  hematite  (specimen  13),  containing 
70  per  cent,  of  another  heavy  metal,  iron  —  with 
quartz  (specimen  15),  which  is  composed  in  nearly 
equal  parts  of  oxygen  and  silicon,  two  typical  non- 
metallic  elements.  Many  minerals  contain  water, 
i.e.,  are  hydrated.  Now  water,  whether  we  consider 
the  liquid  or  solid  state,  is  one  of  the  lightest  and 
softest  of  mineral  constituents  ;  and  it  is  a  very  impor- 
tant fact  that  hydrated  minerals  are  invariably  lighter 
and  usually  softer  than  anhydrous  species  of  otherwise 
similar  composition.  Other  striking  illustrations  of  this 
principle  will  be  pointed  out  in  the  descriptions  of  the 
minerals  which  follow. 

2.  Crystalline  form.  —  A  crystal  is  bounded  by  plane 
surfaces  symmetrically  arranged  with  reference  to  cer- 
tain imaginary  lines  passing  through  its  centre  and 
called  axes.  Crystals  of  the  same  species  are  always 
constant  in  the  angles  between  like  planes,  while  simi- 
lar angles  usually  vary  in  different  species  ;  so  that  each 
species  has  its  own  peculiar  form. 

"  Besides  external  symmetry  of  form,  crystallization 
produces  also  regularity  of  internal  structure,  and  often 
of  fracture.  This  regularity  of  fracture,  or  tendency  to 
break  or  cleave  along  certain  planes,  is  called  cleavage. 
The  surface  afforded  by  cleavage  is  often  smooth  and 
brilliant  (see  specimens  17,  18,  and  21),  and  is  always 
parallel  with  some  external  plane  of  the  crystal.  It 
should  be  understood  that  the  cleavage  lamellae  are 
not  in  any  sense  present  before  they  are  made  to  ap- 
pear by  fracture." —  (Dana.) 


LITHOLOGY.  41 

Crystals  are  arranged  in  six  systems,  based  upon  the 
number  and  relations  of  the  axes,  as  follows  :  — 

Isometric  System.  —  Three  equal  axes  crossing  at 
right  angles.  Example,  cube. 

Tetragonal  System.  —  Two  axes  equal,  third  un- 
equal, all  crossing  at  right  angles.  Example,  square 
prism. 

Orthorhombic  System.  —  Three  unequal  axes,  but 
intersections  all  at  right  angles.  Example,  rhombic 
prism. 

Monoclinic  System.  —  Three  unequal  axes,  one 
intersection  oblique.  Example,  oblique  rhombic 
prism. 

Triclinic  System.  —  Three  unequal  axes,  all  cross- 
ing obliquely.  Example,  oblique  rhomboidal  prism. 

Hexagonal  System.  —  Three  equal  axes  lying  in 
one  plane  and  intersecting  at  angles  of  60°,  and  a 
fourth  axis  crossing  each  of  these  at  right  angles  and 
longer  or  shorter.  Example,  hexagonal  prism. 

By  the  truncation  and  bevelment  of  the  angles  and 
edges  of  these  fundamental  forms  a  vast  variety  of  sec- 
ondary forms  are  produced.  The  limits  of  the  guide 
will  not  permit  us  to  follow  this  topic  farther ;  but  it 
may  be  added  that  for  the  proper  elucidation  of  even 
the  simpler  crystalline  forms  the  teacher  should  'be 
provided  with  a  set  of  wooden  crystal  models  and 
Dana's  "  Text-Book  of  Mineralogy." 

The  crystallization  of  a  mineral  may  be  manifested 
in  two  ways  :  first,  by  the  regularity  of  its  internal  struc- 
ture-or  molecular  arrangement,  as  shown  by  cleavage 
and  the  polarization  of  transmitted  light ;  and,  second, 
by  the  regularity  of  external  form  which  follows,  undef 


42  STRUCTURAL    GEOLOGY. 

favorable  conditions,  as  a  necessary  consequence  of 
symmetry  in  the  arrangement  of  the  molecules. 

When  a  mineral  is  entirely  devoid  of  crystalline 
structure,  both  externally  and  internally,  it  is  said  to  be 
amorphous. 

Perfect  and  distinct  crystals  are  the  rare  exception, 
most  mineral  specimens  being  simply  aggregates  of  im- 
perfect crystals.  In  such  cases,  and  when  the  min- 
eral is  amorphous,  the  structure  of  the  mass  may 
be:  — 

Columnar  or  fibrous. 

Lamellar,  foliaceous,  or  micaceous. 

Granular.  —  When  the  grains  or  crystalline  par- 
ticles are  invisible  to  the  naked  eye  the  mineral  is 
called  impalpable,  compact,  or  massive. 

And  the  external  form  of  the  mass  may  be  :  — 

Botryoidal,  having  grape-like  surfaces. 

Stalactitic,  forming  stalactites  or  pendant  columns. 

Amygdaloidal  or  Concretionary,  forming  separate 
globular  masses  in  the  enclosing  rock. 

Dendritic,  branching  or  arborescent. 

3.  Hardness.  —  By  the  hardness  of  a  mineral  we 
mean  the  resistance  which  it  offers  to  abrasion.  But 
hardness  is  a  purely  relative  term,  calcite,  for  example, 
being  hard  compared  with  talc,  but  very  soft  com- 
pared with  quartz.  Hence  mineralogists  have  found  it 
necessary  to  select  certain  minerals  to  be  used  as  a 
standard  of  comparison  for  all  others,  and  known  as 
the  scale  of  hardness.  These  are  arranged  at  nearly 
equal  intervals  all  the  way  from  the  softest  mineral  to 
the  hardest,  as  follows  :  — 


LITHOLOGY.  43 

Scale  of  Hardness. 

(1)  Talc.  .  (4)  Fluorite.  (8)  Topaz  or  Beryl 

(2)  Gypsum.  (5)  Apatite.  (9)  Corundum. 

(3)  Calcite.  (6)  Orthoclase.  (10)  Diamond. 

(7)  Quartz. 

If  a  mineral  scratches  calcite  and  is  scratched  by 
fluorite,  we  say  its  hardness  is  between  3  and  4,  per- 
haps 3.5  ;  if  it  neither  scratches  nor  is  scratched  by 
orthoclase,  its  hardness  is  6  ;  and  so  on.  There  are 
very  few  minerals  harder  than  quartz,  and  hence  the 
first  seven  members  of  the  scale  are  sufficient  for  all 
ordinary  purposes ;  and  these  are  all  included  in  the 
series  of  specimens  accompanying  this  Guide. 

Although  it  is  desirable  to  be  acquainted  with  the 
scale  of  hardness,  and  to  understand  how  to  use  it,  still 
the  student  will  learn,  after  a  little  practice,  that  almost 
as  good  results  may  be  obtained  much  more  conven- 
iently by  the  use  of  his  thumb-nail  and  a  good  knife- 
blade  or  file.  Talc  and  gypsum  are  easily  scratched 
with  the  nail ;  calcite  and  fluorite  yield  easily  to  the 
knife  or  file,  apatite  with  more  difficulty ;  while  ortho- 
clase is  near  the  limit  of  the  hardness  of  ordinary  steel, 
and  quartz  is  entirely  beyond  it. 

4.  Specific  Gravity.  —  The  specific  gravity  of  a 
mineral,  by  which  we  mean  its  weight  as  compared 
with  the  weight  of  an  equal  volume  of  water,  is  deter- 
mined by  weighing  it  first  in  air  and  then  in  water,  and 
dividing  the  weight  in  air  by  the  difference  of  the  two 
weights.  Minerals  exhibit  a  wide  range  in  specific 
gravity ;  from  petroleum,  which  floats  on  water,  to  gold, 


44  STRUCTURAL    GEOLOGY. 

which  is  nearly  twenty  times  heavier  than  water.  Al- 
though this  is  one  of  the  most  important  properties  of 
minerals,  yet,  being  more  difficult  to  measure  than 
hardness,  it  is  less  valuable  as  an  aid  in  distinguishing 
species.  One  can  with  practice,  however,  estimate  the 
density  of  a  mineral  pretty  closely  by  lifting  it  in  the 
hand. 

5 .  Lustre.  —  Of  all  the  properties  of  minerals  de- 
pending on  their  relations  to  light  the  most  important 
is  lustre,  by  which  we  mean  the  quality  of  the  light  re- 
flected by  a  mineral  as  determined  by  the  character  or 
minute  structure  of  its  surface.     Two  kinds  of  lustre, 
the  metallic  and  vitreous,  are  of  especial  importance ; 
in  fact  all  other  kinds  are  merely  varieties  of  these. 

The  metallic  lustre  is  the  lustre  of  all  true  metals,  as 
copper  and  tin,  and  characterizes  nearly  all  minerals  in 
which  metallic  elements  predominate.  The  vitreous 
lustre  is  best  exemplified  in  glass,  but  belongs  to  most 
minerals  composed  chiefly  of  non- metallic  elements. 
Metallic  minerals  are  always  opaque,  but  vitreous  min- 
erals are  often  transparent. 

Other  kinds  of  lustre  are  the  adamantine  (the  lustre 
of  diamond),  resinous,  pearly,  and  silky.  When  a 
mineral  has  no  lustre,  like  chalk,  it  is  said  to  be  dull. 

It  should  be  made  clear  to  children  that  lustre  and 
color  are  entirely  distinct  and  independent.  Thus, 
iron,  copper,  gold,  silver,  and  lead  are  all  metallic ; 
while  white  or  colorless  quartz,  black  tourmaline,  green 
beryl,  red  garnet,  etc.,  are  all  vitreous.  Generally 
speaking,  any  color  may  occur  with  any  lustre. 

6.  Color  and  Streak.  —  The  colors  of  minerals  are 
of  two  kinds,  —  essential  and   non-essential.     By  the 


LITHOLOGY.  45 

essential  color  in  any  case  we  mean  the  color  of 
the  mineral  itself  in  its  purest  state.  The  non-essential 
colors,  on  the  other  hand,  are  chiefly  the  colors  of  the 
impurities  contained  in  the  minerals. 

Metallic  minerals,  which  are  always  opaque,  usually 
have  essential  colors  ;  but  vitreous  minerals,  which  are 
always  more  or  less  transparent,  often  have  non-essen- 
tial colors.  The  explanation  is  this :  In  opaque  min- 
erals we  can  only  see  the  impurities  immediately  on  the 
surface,  and  these  are,  as  a  rule,  not  enough  to  affect 
its  color ;  but  in  diaphanous  minerals  we  look  into  the 
specimen  and  see  impurities  below  the  surface,  and 
thus  bring  into  view,  in  many  cases,  sufficient  impurity 
so  that  its  color  drowns  that  of  the  mineral. 

To  prove  this  we  have  only  to  take  any  mineral  (ser- 
pentine is  a  good  example  in  our  series)  having  a  non- 
essential  color,  and  make  it  opaque  by  pulverizing  it  or 
abrading  its  surface,  when  the  non-essential  color,  the 
color  of  the  impurity,  immediately  disappears ;  just  as 
water,  yellow  with  suspended  clay,  becomes  white  when 
whipped  into  foam,  and  thus  made  opaque. 

What  we  understand  by  the  streak  of  a  mineral  is  its 
essential  color,  the  color  of  its  powder ;  and  it  is  so 
called  because  the  powder  is  most  readily  observed  by 
scratching  the  surface  of  the  mineral,  and  thereby  pul- 
verizing a  minute  portion  of  it.  The  streak  and  hard- 
ness are  thus  determined  at  the  same  time.  The  streak 
of  soft  minerals  is  easily  determined  by  rubbing  them 
on  any  white  surface  of  suitable  hardness,  as  paper, 
porcelain,  or  Arkansas  stone. 

ESSENTIAL  AND  ACCESSORY  MINERALS.  —  Lithologists, 
regarding  minerals  as  constituents  of  rocks,  divide  them 


46  STRUCTURAL    GEOLOGY. 

into  two  great  classes  :  the  essential  and  the  accessory 
The  essential  constituents  of  a  rock  are  those  mineral* 
which  are  essential  to  the  definition  of  the  rock.  For 
example,  we  cannot  properly  define  granite  without 
naming  quartz  and  orthoclase  ;  hence  these  are  essen- 
tial constituents  of  granite  ;  and  if  either  of  these  min- 
erals were  removed  from  granite  it  would  not  be 
granite  any  longer,  but  some  other  rock.  But  other 
minerals,  like  tourmaline  and  garnet,  may  be  indiffer- 
ently present  or  absent ;  it  is  granite  still ;  hence  they 
are  merely  accidental  or  accessory  constituents.  They 
determine  the  different  varieties  of  granite,  while  the 
essential  minerals  make  the  species. 

This  classification,  of  course,  is  not  absolute,  for  in 
many  cases  the  same  mineral  forms  an  essential  con- 
stituent of  one  rock  and  an  accessory  constituent  of 
another.  Thus,  quartz  is  essential  in  granite,  but  ac- 
cessory in  diorite. 

PRINCIPAL  MINERALS  CONSTITUTING  ROCKS.  —  Having 
studied  in  a  general  way  the  more  important  charac- 
teristics of  minerals,  brief  descriptions  of  the  chief 
rock-forming  species  are  next  in  order.  We  will  notice 
first  and  principally  those  minerals  occurring  chiefly  as 
essential  constituents  of  rocks. 

i .  Graphite.  —  Essentially  pure  carbon,  though  often 
mixed  with  a  little  iron  oxide.  Crystallizes  in  hexago- 
nal system,  but  usually  foliated,  granular,  or  massive. 
Hardness,  1-2,  being  easily  scratched  with  the  nail. 
Sp.  gr.,  2.1-2.3.  Lustre,  metallic ;  an  exception  to  the 
rule  that  acidic  elements  have  non-metallic  or  vitreous 
lustres.  Streak,  black  and  shining  (see  pencil-mark  on 
white  paper).  Color,  iron-black.  Slippery  or  greasy 


LITHOLOGY.  47 

feel.      Every    black-lead    pencil    is   a   specimen    of 
graphite.     Specimen  9. 

The  different  kinds  of  mineral  coal  are,  geologically, 
as  we  have  seen,  closely  related  to  graphite,  but  they 
are  such  familiar  substances  that  they  need  not  be  de- 
scribed here. 

2.  Halite  (common  salt).  —  Chloride  of  sodium: 
chlorine,  60.7;  sodium,  39.3;  =100.    Isometric  sys- 
tem, usually  forming  cubes.     Hardness,  2.5,  a   little 
harder  than  the  nail.     Sp.  gr.,  2.1-2.6.     Lustre,  vit- 
reous.    Streak  and  color  both  white,  and  hence  color 
is  essential.    Often  transparent.    Soluble  ;  taste,  purely 
saline.     In  specific  gravity  and  lustre  it  is  a  good  ex- 
ample of  a  mineral  in  which  an  acidic  element  pre- 
dominates.    Specimen  n. 

3.  Limonite.— Hydrous  sesquioxide  of  iron  :  oxygen, 
25  ;  iron,  60  ;  water,  15  ;  =  100.    Usually  amorphous  ; 
occurring  in  stalactitic  and  botryoidal  forms,  having  a 
fibrous  structure  ;  and  also  concretionary,  massive,  and 
earthy  (yellow  ochre) .   Hardness,  5-5.5.   Sp.  gr.,  3.6-4. 
Lustre,  vitreous  or  silky,  inclining  to  metallic,  and  some- 
times dull.     Color,  various  shades  of  black,  brown,  and 
yellow.     Streak,  ochre-yellow ;  hence  color  partly  non- 
essential.     Specimen  12. 

4.  Hematite.  —  Sesquioxide  of  iron:    oxygen,  30; 
iron,  70 ;  =  100.    Hexagonal  system,  in  distinct  crystals, 
but  usually  lamellar,  granular,  or  compact,  —  columnar, 
botryoidal,  and  stalactitic  forms  being  common.    Hard- 
ness, 5.5-6.5  ;  good  crystals  are  harder  than  steel.    Sp. 
gr.,  4.5-5.3.     Lustre,  metallic,  sometimes  dull.    Color, 
iron-black,  but  red  when    earthy  or  pulverized  (red 
ochre) .     Streak,  red,  and  color,  therefore,  mainly  non 


48  STRUCTURAL    GEOLOGY. 

essential ;  sometimes  attracted  by  the  magnet.  Speci- 
men 13. 

Hematite  has  the  same  composition  as  limontte, 
minus  the  water ;  and  by  comparing  the  hardness  and 
specific  gravity  of  these  two  minerals  we  see  that  they 
are  a  good  illustration  of  the  principle  that  hydrous 
minerals  are  softer  and  lighter  than  anhydrous  minerals 
of  analogous  composition.  Limonite  and  hematite  are 
two  great  natural  coloring  agents,  and  almost  all  yel- 
low, brown,  and  red  colors  in  rocks  and  soils  are  due 
to  their  presence. 

5.  Magnetite.  —  Protoxide  and  sesquioxide  of  iron  : 
oxygen,  27.6;  iron,  72.4;  =  100.  Isometric  system, 
usually  in  octahedrons  or  dodecahedrons.  Most  abun- 
dant variety  is  coarsely  to  finely  granular,  sometimes 
dendritic.  Hardness,  5.5-6.5,  same  as  hematite.  Sp. 
gr.,  4.9-5.2.  Lustre,  metallic.  Color  and  streak,  iron- 
black,  and  hence  color  essential.  Strongly  magnetic  ; 
some  specimens  have  distinct  polarity,  and  are  called 
loadstones.  Specimen  14. 

The  three  iron-oxides  just  described  —  limonite, 
hematite,  and  magnetite  —  are  all  important  ores  of 
iron,  and  form  a  well-marked  natural  series.  Thus 
limonite  is  never,  hematite  is  usually,  and  magnetite  is 
always,  crystalline.  Again,  limonite  with  60  per  cent, 
of  iron  is  never  magnetic,  hematite  with  70  per  cent, 
is  sometimes  magnetic,  while  magnetite  with  72.4  per 
cent,  is  always  magnetic.  As  the  iron  increases  so 
does  the  magnetism.  We  have  here  an  excellent 
illustration  of  the  principle  that  the  properties  of  the 
elements  can  be  traced  in  those  minerals  in  which  they 
predominate.  Iron  is  the  only  strongly  magnetic 


LIT  HO  LOGY.  49 

element :  magnetite  contains  more  iron  than  any  other 
mineral,  and  it  is  the  only  strongly  magnetic  mineral. 

These  three  iron-ores  are  easily  distinguished  from 
each  other  by  the  color  of  their  powders  or  streak,  — 
limonite  yellow,  hematite  red,  and  magnetite  black,  — 
and  from  all  other  common  minerals  by  their  high 
specific  gravity. 

6.  Quartz.  —  Oxide  of  silicon  or  silica :  oxygen, 
53.33  ;  silicon,  46.67  ;  =  100.  Hexagonal  system.  The 
most  common  form  is  a  hexagonal  prism  terminated 
by  a  hexagonal  pyramid.  Also  coarsely  and  finely 
granular  to  perfectly  compact,  like  flint ;  the  compact 
or  cryptocrystalline  varieties  often  assuming  botryoidal, 
stalactitic,  and  concretionary  forms.  It  has  no  cleav- 
age, but  usually  breaks  with  an  irregular,  conchoidal 
fracture  like  glass.  Hardness,  7,  being  No.  7  of  the 
scale  ;  scratches  glass  easily.  Sp.  gr.,  2.5-2.8.  Lustre, 
vitreous.  Pure  quartz  is  colorless  or  white,  but  by 
admixture  of  impurities  it  may  be  of  almost  any  color. 
Streak  always  white  or  light  colored.  Quartz  is  usually, 
as  in  specimen  15,  transparent  and  glassy,  but  may  be 
translucent  or  opaque.  It  is  almost  absolutely  infusible 
and  insoluble. 

The  varieties  of  quartz  are  very  numerous,  but  they 
may  be  arranged  in  two  great  groups  :  — 

1.  Pheno crystalline  or  vitreous  varieties,  including 
rock-crystal,  amethyst,  rose  quartz,  yellow  quartz,  smoky 
quartz,  milky  quartz,  ferruginous  quartz,  etc. 

2.  Cryptocrystalline  or  compact  varieties,  including 
chalcedony,  carnelian,  agate,  onyx,  jasper,  flint,  chert, 
etc.     Only  three  varieties,  however,  are  of  any  great 
geological   importance  ;    these  are  :    common  glassy 
quartz  (spec.  15),  flint  (spec.  16),  and  chert. 


50  STRUCTURAL    GEOLOGY. 

Quartz  is  one  of  the  most  important  constituents 
of  the  earth's  crust,  and  it  is  also  the  hardest  and 
most  durable  of  all  common  minerals.  We  have 
already  observed  (p.  12)  that  it  is  entirely  unaltered 
by  exposure  to  the  weather ;  i.e.,  it  cannot  be  decom- 
posed ;  and,  being  very  hard,  the  same  mechanical 
wear  which,  assisted  by  more  or  less  chemical  decom- 
position, reduces  softer  minerals  to  an  impalpable 
powder  or  clay,  must  leave  the  quartz  chiefly  in  the 
form  of  sand  and  gravel.  This  agrees  with  our  obser- 
vation that  sand  (spec.  30) ,  especially,  is  usually  merely 
pulverized  quartz. 

Opal  is  a  mineral  closely  allied  to  quartz,  and  may 
be  mentioned  in  this  connection.  It  is  of  similar  com- 
position, but  contains  from  5  to  20  per  cent,  of 
water,  and  is  decidedly  softer  and  lighter.  Hardness, 
5.5-6.5  ;  sp.  gr.,  1.9-2.3. 

7.  Gypsum.  —  Hydrous  sulphate  of  calcium  :  sulphur 
trioxide  (SO3),46.5  ;  lime  (CaO),  32.6;  water  (H2O), 
20.9  ;  =  100.     Monoclinic  system.     Often  in  distinct 
rhombic  crystals  ;  also  foliated,  fibrous,  and  finely  gran- 
ular.     Hardness,   1.5-2  ;    the  hardest  varieties  being 
No.  2  of  the  scale  of  hardness.     Sp.  gr.,  2.3.     Lustre, 
pearly,  vitreous,  or  dull.   Color  and  streak  usually  white 
or  gray.  The  principal  varieties  of  gypsum  are  (a)  sele- 
nite,  which  includes  all  distinctly  crystallized  or  trans- 
parent gypsum;    (b)  fibrous   gypsum    or   satin-spar ; 
(c)  alabaster,  fine-grained,  light-colored,  and  translu- 
cent.    Gypsum  is  easily  distinguished  from  all  com- 
mon minerals  resembling  it  by   its  softness   and  the 
fact  that  it  is  not  affected  by  acids'.     Specimen  1 7, 

8.  Calcite.  —  Carbonate  of  calcium  :  carbon  dioxide 


LITHOLOGY.  51 

(C  O2),  44  ;  lime  (CaO),  56  ;  =  100.  Hexagonal  system, 
usually  in  rhombohedrons,  scalenohedrons,  or  hexago- 
nal prisms.  Cleavage  rhombohedral  and  highly  perfect 
(specimen  18).  Also  fibrous  and  compact  to  coarsely 
granular,  in  stalactitic,  concretionary,  and  other  forms. 
Hardness,  2.5-3.5,  usually  3  (see  scale  of  hardness). 
Sp.  gr.,  2.5-2.75.  Lustre,  vitreous.  Color  and  streak 
usually  white.  Transparent  crystallized  calcite  is 
known  as  Iceland-spar,  and  is  remarkable  for  its  strong 
double  refraction.  When  finely  fibrous  it  makes  a 
satin-spar  similar  to  gypsum.  Geologically  speaking, 
calcite  is  a  mineral  of  the  first  importance,  being  the 
sole  essential  constituent  of  all  limestones.  It  is 
readily  distinguished  from  allied  species  by  its  perfect 
rhombohedral  cleavage ;  by  its  softness,  being  easily 
scratched  with  a  knife  ;  and  above  all  by  its  lively  effer- 
vescence with  acids,  for  it  is  the  only  common  mineral 
effervescing  freely  with  cold  dilute  acid.  To  apply  this 
test  it  is  only  necessary  to  touch  the  specimen  with  a 
drop  of  dilute  chlorohydric  acid.  The  effervescence, 
of  course,  is  due  to  the  escape  of  the  carbon  dioxide 
in  a  gaseous  form.  Specimen  18. 

9.  Dolomite.  —  Carbonate  of  calcium  and  magne- 
sium :  carbonate  of  calcium  (CaCO3),  54.35  ;  carbon- 
ate of  magnesium  (MgCO3),45.65  ;  =100.  Hexagonal 
system,  being  nearly  isomorphous  with  calcite.  Rhom- 
bohedral cleavage  perfect.  Hardness,  3.5-4  ;  sp.  gr., 
2.8-2.9,  being  harder  and  heavier  than  calcite.  Lus- 
tre, color,  and  streak  same  as  for  calcite,  from  which  it 
is  most  easily  distinguished  by  its  non -effervescence  or 
only  feeble  effervescence  with  cold  dilute  acid,  though 
effervescing  freely  with  strong  or  hot  acid.  Spec.  19. 


52  STRUCTURAL    GEOLOGY. 

10.  Siderite.  —  Carbonate  of  iron  :  carbon  dioxide 
(C  O2),  37.9  ;  protoxide  of  iron  (Fe  O),  62.1  ;  =  100. 
Crystallization  and  cleavage  essentially  the  same  as  for 
calcite  and  dolomite.  Hardness,  3.5-4.5,  and  sp.  gr., 
3.7-3.9.  Lustre,  vitreous.  Color,  white,  gray,  and 
brown.  Streak,  white.  With  acid,  siderite  behaves  like 
dolomite.  It  is  distinguished  from  both  calcite  and 
dolomite  by  its  high  specific  gravity,  which  is  easily 
explained  by  the  fact  that  it  is  largely  composed  of  the 
heavy  element,  iron. 

With  one  exception,  the  fifteen  minerals  which  we 
have  yet  to  study  belong  to  the  class  of  silicates,  which 
includes  more  than  one-fourth  of  the  known  species  of 
minerals,  and,  omitting  quartz  and  calcite,  all  of  the 
really  important  rock-constituents.  The  silicate  min- 
erals may  be  very  conveniently  divided  into  two  great 
groups,  the  basic  and  acidic.  This  is  not  a  sharp 
division ;  on  the  contrary,  there  is  a  perfectly  gradual 
passage  from  one  group  to  the  other ;  and  yet  this  is, 
for  geological  purposes  at  least,  a  very  natural  classifi- 
cation. The  dividing  line  falls  in  the  neighborhood  of 
60  per  cent,  of  silica ;  i.e.,  all  species  containing  this 
proportion  of  silica  or  less  are  classed  as  basic,  since 
in  them  the  basic  elements  predominate  ;  while  those 
containing  more  than  60  per  cent,  of  silica  are  classed 
as  acidic,  because  their  characteristics  are  determined 
chiefly  by  the  acid  element  or  silica.  The  principal 
bases  occurring  in  the  silicates,  named  in  the  order  of 
their  relative  importance,  are  aluminum,  magnesium, 
calcium,  iron,  sodium,  and  potassium ;  and  of  these, 
magnesium,  calcium,  iron,  and  usually  sodium,  are 
especially  characteristic  of  basic  species. 


LITHOLOGY.  53 

Iron  is  the  heaviest  base  ;  but  all  the  bases,  except 
sodium  and  potassium,  are  heavier  than  the  acid  — 
silica ;  consequently  basic  minerals  must  be,  as  a  rule, 
heavier  than  acidic  minerals.  And  since  basic  mine- 
rals contain  more  iron  than  acidic,  they  must  be  darker 
colored.  In  general,  we  say,  dark,  heavy  silicates  are 
basic,  and  vice  versa.  All  this  is  of  especial  importance 
because  in  the  rocks  nature  keeps  these  two  classes 
separate  in  a  great  degree. 

1 1 .  Amphibole.  —  Silicate  of  aluminum,  magnesium, 
calcium,  iron,  and  sodium.     The  bases  occur  in  very 
various  proportions,  forming  many  varieties ;  but  the 
only  variety  of  especial  geological   interest   is  horn- 
blende, the  average  percentage  composition  of  which 
is  as  follows  :  silica  (SiO2),  50;  alumina  (A12O3),  10  ; 
magnesia  (MgO),  18;  lime   (CaO),  12;   iron  oxide 
(FeO  and  Fe2O3),  8;  and  soda  (Na2O),  2;  =  100. 
Monoclinic  system  :    usually  in  rhombic  or  six-sided 
prisms  which  may  be  short  and  thick,  but  are  more 
often  acicular  or  bladed.      Hardness,   5-6 ;    sp.  gr., 
2.9-3.4.     Lustre,  vitreous ;  color,  black  and  greenish 
black ;    and  streak  similar  to  color,  but  much  paler. 
Compare  with  quartz,  and  observe  the  strong  contrast 
in  color  possible  with  minerals  having  the  same  lustre. 
Specimen  20. 

12.  Pyroxene.  —  Like  amphibole,  this  species  em- 
braces many  varieties,  and  these  exhibit  a  wide  range 
in  composition ;  but  of  these  augite  alone  is  an  impor- 
tant rock-constituent.     Hence  in  lithology  we  practi- 
cally substitute   for  amphibole    and   pyroxene,  horn- 
blende, and  augite  respectively. 

Augite  is  very  similar  in  composition  to  hornblende, 


54  STRUCTURAL    GEOLOGY. 

but  contains  usually  more  lime  and  less  alumina  and 
alkali.  Physically,  too,  these  minerals  are  almost 
identical,  crystallizing  in  the  same  system  and  in  very 
similar  forms,  and  agreeing  in  hardness,  color,  lustre, 
and  streak.  Augite  is  heavier  than  hornblende,  sp.  gr., 
3.2-3.5.  A  certain  prismatic  angle,  which  in  augite 
is  87°5f,  is  i24°3o'  in  hornblende.  Slender,  bladed 
crystals  are  more  common  with  hornblende  than  augite. 
When  examined  in  thin  sections  with  the  polarizer, 
augite  does  not  afford  the  phenomenon  of  dichroism, 
which  is  strongly  marked  in  hornblende.  However,  as 
these  minerals  commonly  occur  in  the  rocks,  in  small 
and  imperfect  crystals,  these  distinctions  can  only  be 
observed  in  thin  sections  under  the  microscope ;  so 
that,  as  regards  the  naked  eye,  they  are  practically 
indistinguishable. 

It  might  appear  at  first  that  the  distinction  of  mine- 
rals so  nearly  identical  is  not  an  important  matter ;  but 
nature  has  decreed  otherwise.  Augite  and  hornblende 
are  typical  examples  of  basic  minerals ;  but  augite  is, 
both  in  its  composition  and  associations,  the  more 
basic  of  the  two.  In  proof  of  this  we  need  only  to 
know  that  it  very  rarely  occurs  in  the  same  rock  with 
quartz,  while  hornblende  is  found  very  commonly  in 
that  association.  Quartz  in  a  rock  means  an  excess  of 
acid  or  silica,  and  almost  necessarily  implies  the  absence 
of  highly  basic  minerals.  In  other  words,  hornblende 
is  often,  and  augite  very  rarely,  found  in  connection  with 
acidic  minerals ;  and  it  is  this  difference  of  association 
chiefly  that  makes  their  distinction  essential  to  the 
proper  recognition  of  rocks ;  while  at  the  same  time 
it  affords  an  easy,  though  of  course  not  absolutely  cer- 


LITHOLOGY.  55 

tain,  means  of  determining  whether  the  black  constit- 
uent of  any  particular  rock  is  hornblende  or  augite. 

Mica  Family.  —  Mica  is  not  the  name  of  a  single 
mineral,  but  of  a  whole  family  of  minerals,  including 
some  half-dozen  species.  Only  two,  however,  —  mus- 
covite  and  biotite,  —  are  sufficiently  abundant  to  engage 
our  attention.  These  are  complex,  basic  silicates  of 
aluminum,  magnesium,  iron,  potassium,  and  sodium. 
The  crystallization  of  biotite  is  hexagonal,  and  of  mus- 
covite  monoclinic ;  but  both  occur  commonly  in  flat 
six-sided  forms.  Undoubtedly  the  most  important  and 
striking  characteristic  of  the  whole  mica  family  is  the 
remarkably  perfect  cleavage  parallel  with  the  basal 
planes  of  the  crystals,  and  the  wonderful  thinness,  and 
above  all  the  elasticity,  of  the  cleavage  lamellae.  The 
cleavage  contrasts  the  micas  with  all  other  common 
minerals,  and  makes  their  certain  identification  one  of 
the  easiest  things  in  lithology.  The  micas  are  soft 
minerals,  the  hardness  ranging  from  2  to  3,  and  being 
usually  easily  scratched  with  the  nail.  Sp.  gr.  varies 
from  2.7-3.1.  Lustre,  pearly;  and  streak,  white  or 
uncolored. 

The  distinguishing  features  of  muscovite  and  biotite 
are  as  follows  :  — 

13.  Muscovite.  —  Contains  47  per  cent,  of  silica,  3 
per  cent,  of  sesquioxide  of  iron,  and  10  per  cent,  of 
alkalies,  chiefly  potash ;   and  the  characteristic  colors 
are  white,  gray,  and,  more  rarely,  brown  and  yellow. 
Non-dichroic.    Usually  found  in  association  with  acidic 
minerals.     The  mica  used  in  the  arts  is  muscovite. 
Specimen  21. 

14.  Biotite.  —  Contains  only  36  per  cent,  of  silica. 


56  STRUCTURAL    GEOLOGY. 

20  per  cent,  of  oxide  of  iron,  and  17  per  cent,  of  mag- 
nesia ;  colors,  deep  black  to  green.  Strongly  dichroic. 
Commonly  occurs  with  other  basic  minerals.  Com- 
pare color  with  per  cent,  of  iron. 

These  differences  are  tabulated  below  :  — 

Muscovite •  =  Biotite  — 

Acidic  mica.  Basic  mica. 

Non-ferruginous  mica.  Ferruginous  mica. 

Potash  mica.  Magnesian  mica. 

White  mica.  Black  mica. 

Non-dichroic  mica.  Dichroic  mica. 

Feldspar  Family.  —  Like  mica,  feldspar  is  the  name 
of  a  family  of  minerals  ;  and  these  are,  geologically,  the 
most  important  of  all  minerals.  They  are,  above  all 
others,  the  minerals  of  which  rocks  are  made,  and  their 
abundance  is  well  expressed  in  the  name,  —  feldspar 
being  simply  the  German  for  field-spar,  implying  that 
it  is  the  common  spar  or  mineral  of  the  fields. 

Chemically,  the  feldspars  are  silicates  of  aluminum 
and  potassium,  sodium  or  calcium.  They  crystallize 
in  the  monoclinic  and  triclinic  systems ;  and  all  pos- 
sess easy  cleavage  in  two  directions  at  right  angles  to 
each  other,  or  nearly  so.  The  general  physical  char- 
acters, including  the  cleavage,  are  well  exhibited  in  the 
common  species,  orthoclase  (specimen  22). 

In  hardness  the  feldspars  range  from  5  to  7,  being 
usually  near  6,  and  almost  always  distinctly  softer  than 
quartz.  Sp.  gr.  varies  from  2.5-2.75  ;  lustre,  from 
vitreous  to  pearly;  color,  from  white  and  gray  to  red, 
brown,  green,  etc.,  but  usually  light.  Streak,  always 
white  ;  rarely  transparent.  By  exposure  to  the  weather, 
feldspars  gradually  lose  their  alkalies  and  lime,  become 


LITHOLOGY.  57 

hydrated,  and  are  changed  to  kaolin  or  common  clay. 
A  similar  change  takes  place  with  the  micas,  augite, 
and  hornblende  ;  but  these  species,  being  usually  rich  in 
iron,  make  clays  which  are  much  darker  colored  than 
those  derived  from  feldspars.  The  fact  that  the  feld- 
spars contain  little  or  no  iron  undoubtedly  explains 
their  low  specific  gravity  and  light  colors,  as  compared 
with  the  other  minerals  just  named.  The  only  com- 
mon minerals  for  which  the  feldspars  are  liable  to  be 
mistaken  are  quartz  and  the  carbonates.  From  the 
latter  they  are  easily  distinguished  by  their  supertor 
hardness  and  non-effervescence  with  acids ;  and  from 
the  former,  by  possessing  distinct  cleavage,  by  being 
rarely  transparent,  by  being  somewhat  softer,  and  by 
changing  to  clay  on  exposure  to  the  weather. 

The  feldspars  of  greatest  geological  interest  are  five 
in  number,  and  may  be  classified  chemically  as  fol- 
lows :  — 

Orthoclase,  — silicate  of  aluminum  and  potassium,  or 

potash  feldspar. 
A-lbite,          .         "  "          "     sodium,  or 

soda  feldspar. 
Anorthite,  "  "          "     calcium,  or 

lime  feldspar. 
Oligoclase,  "         sodium,  and  calcium,  or 

soda-lime  feldspar. 
Labradorite,          "  "         calcium,  and  sodium,  or 

lime-soda  feldspar. 

This  appears  like  a  complex  arrangement,  but  it  can 
be  simplified.  Orthoclase  crystallizes  in  the  monocli- 
nic  system,  and  all  the  other  feldspars  in  the  triclinic 
system.  With  the  exception  of  albite,  which  is  a  com- 


58  STRUCTURAL   GEOLOGY. 

paratively  rare  species,  the  triclinic  feldspars  all  contain 
less  silica  than  orthoclase  ;  i.e.,  are  more  basic.  This  is 
shown  by  the  subjoined  table  giving  the  average  com- 
position of  each  of  the  feldspars  :  — 

SiO2  A12O3  K2O        Na2O  CaO       Total. 

Orthoclase,          65  18  17  —    =    100 

Albite,                 68  20  12  —    =    100 

Oligoclase,          62  24  9  5    —    100 

Labradorite,        53  30  4  13    =    100 

Anorthite,           43  37  —  20    =    100 

As  we  should  naturally  expect,  the  triclinic  feldspars 
occur  usually  with  other  basic  minerals,  while  the  mono- 
clinic  species,  orthoclase,  is  acidic  in  its  associations ; 
furthermore,  the  triclinic  feldspars  are  often  intimately 
associated  with  each  other,  but  are  rarely  important 
constituents  of  rocks  containing  much  orthoclase.  In 
other  words,  the  distinction  of  orthoclase  from  the  basic 
or  triclinic  feldspars  is  important  and  comparatively 
easy,  while  the  distinction  of  the  different  basic  feld- 
spars from  each  other  is  both  unimportant  and  difficult. 
Hence,  in  lithology,  we  find  it  best  to  put  all  these 
basic  feldspars  together,  as  if  they  were  one  species, 
under  the  name  plagiodase,  which  refers  to  the  oblique 
cleavage  of  all  these  feldspars,  and  contrasts  with 
orthoclase,  which  refers  to  the  right-angled  cleavage  of 
that  species. 

This  statement  of  the  relations  of  the  feldspars  is,  of 
course,  beyond  the  comprehension  of  many  children  j 
and  yet  it  should  be  understood  by  the  teacher  who 
would  lead  the  children  to  any  but  the  most  superficial 
views. 

15.   Orthoclase.  —  This  is  the  common  feldspar,  and 


LITHOLOGY.  59 

the  most  abundant  of  all  minerals,  being  the  principal 
constituent  of  granite,  gneiss,  and  many  other  impor- 
tant rocks.  The  most  characteristic  colors  are  white, 
gray,  pinkish,  and  flesh-red.  Specimen  22. 

1 6.  Plagioclase. —  Like  orthoclase,  these  species 
may  be  of  almost  any  color ;  yet  these  two  great  divi- 
sions of  the  feldspars  are  usually  contrasted  in  this 
respect.  Thus,  bluish  and  grayish  colors  are  most 
common  with  plagioclase,  and  white  or  reddish  colors 
with  orthoclase.  Specimen  23  is  labradorite,  and,  in 
every  respect,  a  typical  example  of  plagioclase.  On 
certain  faces  and  cleavage-surfaces  of  the  plagioclase 
crystals  we  may  often  observe  a  series  of  straight  parallel 
lines  or  bands  which  are  often  very  fine,  —  fifty  to  a 
hundred  in  a  single  crystal.  These  striae  are  due  to 
the  mode  of  twinning,  and  are  of  especial  importance, 
since,  while  they  are  very  characteristic  of  plagio- 
clase, they  never  occur  in  orthoclase.  As  stated,  these 
twinning  striae  in  plagioclase  are  often  visible  to  the 
naked  eye ;  and  when  they  are  not,  they  may  usually 
be  revealed  by  examining  a  thin  section  under  the 
microscope  with  polarized  light.  Plagioclase  decays 
much  more  rapidly  when  exposed  to  the  weather  than 
orthoclase.  This  point  becomes  perfectly  clear  when 
we  compare  weathered  ledges  of  diabase  (or  any  trap- 
rock,  see  specimen  2)  and  granite;  for  plagioclase 
is  the  principal  constituent  of  the  former  rock,  and 
orthoclase  of  the  latter. 

Hydrous  Silicates.  —  Many  silicates  contain  water, 
and  some  of  these  are  of  great  geological  importance. 
What  has  been  stated  on  a  preceding  page  concerning 
the  softness  and  lightness  of  hydrated  minerals  is  espe- 


60  STRUCTURAL    GEOLOGY. 

cially  applicable  here  ;  for  all  the  geologically  important 
hydrous  silicates  are  distinctly  softer  and  lighter  than 
anhydrous  minerals  of  otherwise  similar  composition. 
Furthermore,  they  usually  have  an  unctuous  or  slippery 
feel ;  and,  with  one  exception  (kaolin) ,  are  of  a  green 
or  greenish  color. 

1 7.  Kaolinite  (Kaolin).  —  Hydrous  silicate  of  alum- 
inum :    silica  (SiO2),  46;   alumina  (A12O3),  40;   and 
water  (H2O),  14;   =  100.     Orthorhombic  system,  in 
rhombic  or  hexagonal  scales  or  plates,  but  usually  earthy 
or    clay-like.      Hardness,    1-2.5  >     SP-    grv    2.4-2.65. 
The  pure  mineral  is  white ;  but  it  is  usually  colored 
by  impurities,  the   principal  of  which  are  iron  oxides 
and  carbonaceous  matter.     Kaolin  is  the  most  abun- 
dant of  all  the  hydrous  silicates,  and  it  is  the  basis  and 
often  the  sole  constituent  of  common  clay,  —  a  very 
common  mineral,  but  rarely  pure.     We  have  already 
(p.  u)  noticed  the  mode  of  origin  of  kaolin  or  clay. 
It  results  from  the  decomposition  of  various  aluminous 
silicate  minerals,  especially  the  feldspars.     Under  the 
combined  influence  of  carbon  dioxide  and  moisture, 
feldspars  give  up  their  potassium,  sodium,  and  calcium, 
and  take  on  water,  and  the  result  is  kaolin.     This  min- 
eral is  believed  to  be  always  a  decomposition  product. 
Perhaps  the  best,  or  at  least  the  most  convenient,  test 
for  kaolin  is  the  argillaceous  odor,  the  odor  of  moist- 
ened clay.    Specimen  24. 

1 8.  Talc.  —  Hydrous  silicate  of  magnesium:  silica 
(SiO2),    63    (acidic);    magnesia    (MgO),    32;    water 
(H2O),  5  ;  =  100.    Orthorhombic  system,  but  rarely  in 
distinct  crystals.     Cleavage  in  one  direction  very  per- 
fect •  the  cleavage  lamellae  are  flexible,  but  not  elastic, 


LITHOLOGY.  6 1 

as  in  mica.  Hardness,  i  ;  see  scale.  Sp.  gr.,  2.55-2.8. 
Lustre,  pearly.  Color,  apple-green  to  white  ;  and  streak, 
white.  The  feel  is  very  smooth  and  greasy ;  and,  in 
connection  with  the  color  and  foliation,  affords  the  best 
means  of  distinguishing  talc  from  allied  minerals.  Talc 
sometimes  results  from  the  alteration  of  augite,  horn- 
blende, and  other  minerals,  but  it  is  not  always  nor 
usually  an  alteration  product. 

19.  Serpentine.  —  Hydrous  silicate  of  magnesium : 
silica  (Si  O2),  44  (basic)  ;  magnesia  (Mg  O),  44  ;  water 
(H2O),  12  ;  =100.    Essentially  amorphous.   Hardness, 
2.5-4;    sp.gr.,    2.5-2.65.     Lustre,    greasy,    waxy,    or 
earthy.     Color,  various  shades  of  green  and  usually 
darker   than    talc,    but    streak    always    white.      Feel, 
smooth,   sometimes   greasy.     Distinguished  from  talc 
by    its    hardness,    compactness,    and    darker    green. 
Sometimes  results  from  the  alteration  of  olivine  and 
other  magnesian  minerals,  but  usually  we  are  to  regard 
it  as  an  original  mineral.     Specimen  25. 

20.  Chlorite. — This  is,  properly,   the  name  of  a 
group  of  highly  basic  minerals  of  very  variable  compo- 
sition, but  they  are  all  essentially  hydrous  silicates  of 
aluminum,  magnesium,  and    iron ;    and    the    average 
composition  of  the  most  abundant  species,  prochlorite, 
is  as  follows  :  silica  (SiO2),  30  ;  alumina  (A12O3),  18  ; 
magnesia  (MgO),  15  ;  protoxide  of  iron  (FeO),  26  ; 
and  water  (H.2O),  n  ;  =100.    The  chlorites  crystallize 
in  several  different  systems,  but  in  all  there  is  a  highly 
perfect  cleavage  in  one  direction,  giving,  as  in  talc,  a 
foliated   structure  with  flexible  but  inelastic  laminae. 
The  cleavage  scales,  however,  are  sometimes  minute, 
and  the  structure  massive  or  granular.     Hardness  of 


62  STRUCTURAL    GEOLOGY. 

prochlorite,  1-2  ;  between  talc  and  serpentine.  Sp.gr., 
2.78-2.96.  All  the  chlorites  have  a  pearly  to  vitreous 
lustre.  Color  usually  some  shade  of  green ;  in  pro- 
chlorite a  dark  or  blackish  green,  darker  than  serpentine, 
as  that  is  darker  than  talc.  Streak,  a  lighter,  whitish 
green.  Less  unctuous  than  talc,  but  more  so  than  ser- 
pentine. The  chlorites  are  produced  very  commonly, 
but  not  generally,  by  the  alteration  of  basic  anhydrous 
silicates,  like  augite  and  hornblende.  Specimen  26. 

2 1 .  Hydro-mica.  —  This,  too,  is  properly  the  name 
of  a  group  of  minerals ;  but  for  geological  purposes 
they  may  be  regarded  as  one  species.     Taking  a  gen- 
eral view  of  the  composition,  these  are  simply  the  an- 
hydrous or  ordinary  micas,   which  we    have    already 
studied,  with  from  5  to  10  per  cent,  of  water  added. 
In   crystallization   and    structure    they  are   essentially 
mica-like.      Although    not    distinctly  softer  than   the 
common  micas,  they  are  lighter,  always  more  unctuous 
and  slippery,  and  usually  of  a  greenish  color.     The 
micaceous  structure  with  elastic  laminae  serves  to  dis- 
tinguish the  hydro-micas  from  other  hydrous  silicates. 

22.  Glauconite* — Hydrous    silicate    of  aluminum, 
iron,  and  potassium  :  silica(SiO2),5o  ;  alumina (A12O3), 
protoxide  of  iron  (FeO),  and  potash  (K2O),  together, 
41  ;  and  water  (H2O),9  ;  =  100.  Amorphous,  forming 
rounded  and  generally  loose  grains,  which  often  have 
a  microscopic  organic  nucleus.     It  is  dull  and  earthy, 
like  chalk,  and  always  soft,  green,  and  light,  but  not 
particularly   unctuous.      Glauconite   is    the    principal, 
often  the  sole,  constituent  of  the  rock  greensand,  which 
occurs  abundantly  in  the  newer  geological  formations, 
and  is  now  forming  in  the  deep  water  of  the  Gulf  of 


LITHOLOGY.  63 

Mexico   and   along   our   Atlantic   sea-board.      Speci- 
men 27. 

This  completes  our  list  of  minerals  occurring  chiefly 
as  essential  constituents  of  rocks ;  and  following  are 
three  of  the  more  common  and  important  minerals 
occurring  chiefly  as  accessory,  rarely  as  essential,  rock- 
constituents. 

23.  Chrysolite  (Oiivine).  —  Silicate   of    magnesium 
and  iron:  silica  (SiO2),  41;  magnesia  (MgO),  51; 
protoxide  of  iron  (Fe2O3),  8  ;  =  100.     Orthorhombic 
system ;  but  usually  in  irregular  glassy  grains.     Hard- 
ness, 6-7.     Sp.  gr.,  3.3-3.5.     Lustre,  vitreous ;  color, 
usually   some    shade    of    green ;    and   streak,   white. 
Chrysolite  sometimes  closely  resembles  quartz,  but  its 
green  color  usually  suffices  to  distinguish  it.     It  is  a 
common  constituent  of  basalt  and  allied  rocks.     By 
absorption  of  water  it  is  changed  into  serpentine  and 
talc.     See  examples  in  specimen. 

24.  Garnet.  —  The  composition  of  this  mineral  is 
extremely  variable  ;  but  the  most  important  variety  is  a 
basic  silicate  of  aluminum  and  iron  :  silica  (SiO2),  37  ; 
alumina  (A12O3),  20  ;  and  protoxide  of  iron  (FeO),43  '•> 
=  100.     Isometric  system,  usually  in  distinct  crystals, 
twelve-sided   (dodecahedrons)   and    twenty-four-sided 
(trapezohedrons)  forms  being  most  common.     Hard- 
ness, 6.5-7.5 ;    average    as    hard  as  quartz.     Sp.  gr., 
3.15-4.3 ;  compare  with  the  high  percentage  of  iron. 
Lustre,  vitreous  ;  colors,  various,  usually  some  shade  of 
red  or  brown ;  and  streak,  white.     Some  varieties  con- 
tain iron  enough  to  make  them  magnetic.     Garnet  is 
easily  distinguished  by  its  form,  color,  and  hardness 
from  all  other  minerals.     It  is  a  common  but  not  an 


I 

64  STRUCTURAL  GEOLOGY. 

abundant  mineral,  occurring  most  frequently  in  gneiss, 
mica  schist,  and  other  stratified  crystalline  rocks.  See 
examples  in  specimen. 

25.  Pyrite.  —  Sulphide  of  iron  :  sulphur,  53.3  ;  iron, 
46.7;  =  100.  Isometric  system,  occurring  usually  in 
distinct  crystals,  the  cube  and  the  twelve-sided  form 
known  as  the  pyritohedron  being  the  most  common. 
Hardness,  6-6.5,  striking  fire  with  steel.  Sp.  gr.,  4.8- 
5.2  ;  heavy  because  rich  in  iron.  Lustre,  metallic  and 
splendent.  Color,  pale,  brass-yellow,  and  streak, 
greenish  or  brownish.  Pyrite  is  sometimes  mistaken 
for  gold,  but  it  is  not  malleable ;  while  its  color,  hard- 
ness, and  specific  gravity,  combined,  easily  distinguish 
it  from  all  common  minerals.  As  an  accessory  rock- 
constituent,  pyrite  occurs  usually  in  isolated  cubes  or 
pyritohedrons.  Specimen  10. 

Textures  of  Rocks. 

Texture  is  a  general  name  for  those  smaller  struc- 
tural features  of  rocks  which  can  be  studied  in  hand 
specimens,  and  which  depend  upon  Reforms  and  sizes 
of  the  constituent  particles  of  the  rocks,  and  the  ways 
in  which  these  are  united. 

By  "constituent  particles"  we  mean,  not  the  atoms 
or  molecules  of  matter  composing  the  rocks,  but  the 
pebbles  in  conglomerate,  grains  of  sand  in  sandstone, 
crystals  of  quartz,  feldspar,  and  mica  in  granite,  etc. 
The  four  most  important  textures  are  :  — 

(\)Fragmental  texture. —  The  rock  is  composed  of 
mere  irregular,  angular,  or  rounded,  but  visible,  frag- 
ments. Examples  :  sand,  sandstone,  gravel,  conglom- 
erate, etc.  Specimens  30,  31,  28,  29. 


LITHOLOGY.  65 

(2)  Crystalline  texture.  —  The  constituent  particles 
are  chiefly,  at  least,  distinctly  crystalline,  as  shown 
either  by  external  form,  or  cleavage,  or  both.  Exam- 
ples :  granite,  diabase,  gneiss,  etc.  Specimens  45,  i, 
41. 

(3).  Compact  texture.  —  The  constituent  particles 
are  indistinguishable  by  the  naked  eye,  but  become  visi- 
ble under  the  microscope,  appearing  as  separate  crystal- 
line grains  or  as  irregular  fragments.  In  other  words,  if, 
in  the  case  of  either  the  granular  or  crystalline  textures, 
we  conceive  the  particles  to  become  microscopically 
small,  then  we  have  the  compact  texture.  Examples  : 
clay,  slate,  many  limestones,  basalt,  etc.  Specimens 

34,  35,  39- 

(4  )  Vitreous  texture. — The  texture  of  glass,  in  which 
the  constituent  particles  are  absolutely  invisible  even 
with  the  highest  powers  of  the  microscope,  and  may 
be  nothing  more  than  the  molecules  of  the  substance, 
which  thus,  so  far  as  our  powers  of  observation  are 
concerned,  presents  a  perfectly  continuous  surface. 
Examples  :  obsidian,  glassy  quartz,  and  some  kinds  of 
coal.  Specimens  47,  15. 

These  four  textures,  which,  it  will  be  observed,  are 
determined  by  the  forms  and  sizes  of  the  constituent 
particles,  may  be  called  the  primary  textures,  because 
every  rock  must  possess  one  of  them.  We  cannot  con- 
ceive of  a  rock  which  is  neither  fragmental,  crystalline, 
compact,  nor  vitreous.  But  in  addition  to  one  of  the 
primary  textures,  a  rock  may  or  may  not  have  one  or 
more  of  what  may  be  called  secondary  textures.  These 
are  determined  by  the  way  in  which  the  particles  are 
united,  the  mode  or  pattern  of  the  arrangement,  etc. 


66  STRUCTURAL  GEOLOGY. 

Following  are  definitions  of  the  principal  secondary 
textures :  — 

1 i )  Laminated  texture. — This  exists  where  the  par- 
ticles are  arranged  in  thin,  parallel  layers,  which  may 
be  marked  simply  by  planes  of  division,  or  the  alter- 
nate layers  may  be  composed  of  particles  differing  in 
composition,  form,  size,    or   color,  etc.     Among    the 
laminated  textures  we  thus  distinguish  :  (a)  the  banded 
texture,  where  the  layers  are  contrasted  in  color,  tex- 
ture, or  composition,  but  cohere,  so  that  there  is  no 
cleavage  or  easy  splitting  parallel  with  the  stratification  ; 
and  (b)  the  schistose  or  shaly  texture,  where  such  fissility 
or  stratification-cleavage  exists.     If  a  fragmental,  com- 
pact, or  vitreous  rock  is  fissile,  we  use  the  term  shaly; 
but  a  fissile,  crystalline  rock  is  described  as  schistose. 
The  banded  texture  may  occur  with  the  fragmental,  — 
banded  sandstones,  etc. ;  with  the  crystalline,  —  many 
gneisses,  etc.   (specimen  41);   with  the  compact, — • 
many  slates,  limestones,  felsites,  etc.   (specimens  34, 
42);   with  the  vitreous,  —  banded   obsidian,  furnace 
slags,  and  some  coal.    The  schistose  texture  may  occur 
with  the  crystalline,  —  mica  schist,  etc.  (specimen  43)  ; 
and  the  shaly  texture  with  the  compact  and  fragmental, 
but  rarely  with  the  vitreous. 

(2)  Porphyritic   texture. — We    have   this    texture 
when  separate  and  distinct  crystals  of  any  mineral,  but 
most  commonly  of  feldspar,  are  enclosed  in  a  relatively 
fine-grained  base  or  matrix,  which  may  be  either  crystal- 
line, compact,  or  vitreous,  but  rarely  fragmental.    Speci- 
mens 5,  6,  7  are  examples  of  the  porphyritic  compact 
texture. 


LIT  HO  LOGY.  67 

(3)  Concretionary  texture. — When  one   or  more 
constituents  of  a  rock  have  the  form,  in  whole  or  in  part, 
not  of  distinct  angular  crystals,  but  of  rounded  concre- 
tions, the   texture  is  described  as  concretionary,  the 
concretions  taking  the  place  in  this  texture  of  the  iso- 
lated crystals  in  the  porphyritic  texture.     This  texture 
occurs  in  connection  with  all  the  primary  textures,  but 
the  most  familiar  example  is  oolitic  limestone. 

(4)  Vesicular  texture.  —  A  rock  has  this  texture 
when  it  contains  numerous  small  cavities  or  vesicles. 
These  are  most  commonly  produced  by  the  expansion 
of  steam  and  other  vapors  when  the  rock  is  in  a  plastic 
state  ;  and  hence  the  vesicular  texture  is  found  chiefly 
in  volcanic  rocks.     Except  rarely,  it  is  associated  only 
with  the  compact  texture, — ordinary  stony  lavas  (spec- 
imen 49)  ;  and  with  the  vitreous   texture,  —  pumice 
(specimen  48). 

(5 )  Amygdaloidal  texture.  —  In  the  course  of  time 
the  vesicles  of  common  lava  are  often  filled  with  various 
minerals  deposited  by  infiltrating  waters,  giving  rise  to 
the  amygdaloidal  texture,  from  the  Latin  amygdalum,  an 
almond,  in  allusion  to  a  common  form  of  the  vesicles,  or 
amygdules,  as  they  are  called,  after  being  filled.     The 
amygdaloidal  texture  is  thus  necessarily  preceded  by  the 
vesicular,  and  is  limited  to  the  same  classes  of  rocks. 
Specimen  50. 

Besides  the  foregoing,  there  are  many  minor  sec- 
ondary textures.  The  rocks  known  as  tufas  have  what 
may  be  called  the  tufaceous  texture.  Then  we  have 
kinds  of  texture  depending  on  the  strength  of  the  union 
of  the  particles,  as  strong,  weak,  friable,  earthy,  etc. 


68  STRUCTURAL  GEOLOGY. 

Classification  of  Rocks. 

Having  finished  our  preliminary  observations  on  the 
characteristics  of  rocks,  we  are  now  about  ready  to 
begin  a  systematic  study  of  the  rocks  themselves ;  but 
it  is  needful  first  to  say  a  few  words  about  the  classifi- 
cation of  rocks,  since  upon  this  depends  not  only  the 
order  in  which  we  shall  take  the  rocks  up,  but  also  the 
ideas  that  will  be  imparted  concerning  their  relations 
and  affinities.  The  classifications  which  have  been 
proposed  at  different  times  are  almost  as  numerous  as 
She  rocks  themselves.  Some  of  these  are  confessedly, 
and  even  designedly,  artificial,  as  when  we  classify 
stones  according  to  their  uses  in  the  arts,  etc.  But  we 
want  something  more  scientific,  a  natural  classification  ; 
that  is,  one  based  upon  the  natural  and  permanent 
characteristics  of  rocks.  Rocks  have  been  classified 
according  to  chemical  composition,  mineralogical  com- 
position, texture,  color,  density,  hardness,  etc. ;  but 
these  arrangements,  taken  singly  or  all  combined,  are 
inadequate. 

A  natural  classification  may  be  defined  as  a  concise 
and  systematic  statement  of  the  natural  relations  exist- 
ing among  the  objects  classified.  Now  the  most,  im- 
portant relations  existing  among  rocks  are  those  due 
to  their  different  origins.  We  must  not  forget  that 
lithology  is  a  branch  of  geology,  and  that  geology  is 
first  of  all  a  dynamical  science.  The  most  important 
question  that  can  be  asked  about  any  rock  is,  not  What 
is  it  made  of?  but  How  was  it  made  ?  What  were  the 
general  forces  or  agencies  concerned  in  its  formation? 
Rocks  are  the  material  in  which  the  earth's  history  is 


Classification  of  Rocks, 

Sedimentary  or  Stratified  Rocks. 

MECHANICALLY  FORMED. 

Unconsolidated.                                  Consolidated. 

Conglomerate 
group. 

Gravel.                                          Conglomerate. 

A  renaceous 
group. 

Sand.                                                 Sandstone. 

A  rgillaceous 
group. 

Clay.                                                   Slate. 

CHEMICALLY  AND  ORGANICALLY  FORMED. 

Coal 

group. 

Iron-ore 

group. 

C  ale  ar  tons 
group. 

Me  amorphic  group  (Silicates'), 
Acidic.                        Basic. 

85       80              70              60              SO             40            30 

Peat. 
Lignite. 
Bit.  Coal. 

Anthracite. 
Graphite. 
Asphaltum. 

Limonite. 
Hematite. 
Magnetite. 
Siderite. 

Limestone. 
Dolomite. 
Gypsum. 
Rock-salt. 
Phosphate 
Rock. 

Feldspathic. 

Gneiss.           [  Diorite. 

Syenite.     Norite. 

Siliceous 
group. 

Non-Feldspathic. 

Mica  Schist. 

Tripolite. 
Flint. 
Siliceous 
Tufa. 
Novaculite. 

Hornbl.  Schist.  Amphiboli  e. 

Talc  Schist.         Chi.  Schist. 
i 
Greensand   Serpentine. 

Eruptive  or  Unstratified  Rocks. 

PLUTONIC. 

Thisp 
is  a  blan 
no   erupt 
which  ar 
minerals 

irt  of  the  clas 
k,  for  the  re. 
ive  rocks   ar 
e  chiefly  com 
jelonging  to  t 
a  Elements,  C 
Sulphates,  or 
,  all  eruptive 
nown,    are  pi 
1  of  minerals  V. 
ss  of  Silicates 

sification 
ison  that 
e  known 
posed  of 
ic  classes 
'hlorides 

| 
I 

| 
Granite.           Diori  e. 

Syenite.  D  abase. 

VOLCANIC. 

Oxides, 
ates;  i.e 
far  as   k 
composec 
to  the  cla 

Carbon- 
rocks,  so 
•incipally 
elonging 

Feldspathic. 

Rhyol  te.           Andesite. 

Trachyte.   Basalt. 
Obsidian.            Tachylite. 

Petrosilex.          Porphyrite. 

Felsite.  Melaphyr. 

,  ,-••  •••; 

LITHOLOGY.  71 

written,  and  what  we  want  to  know  first  concerning  any 
rock  is  what  it  can  tell  us  of  the  condition  of  that  part 
of  the  earth  at  the  time  it  was  made  and  subsequently. 

The  geological  agencies,  as  we  have  already  learned, 
may  be  arranged  in  two  great  classes :  first,  the  aqueous 
or  superficial  agencies  originating  in  the  solar  heat,  and 
producing  the  sedimentary  or  stratified  rocks ;  and, 
second,  the  igneous  or  subterranean  agencies  originat- 
ing in  the  central  or  interior  heat,  and  producing  the 
eruptive  or  unstratified  rocks.  Hence,  we  want  to 
know  first  of  any  rock  whether  it  is  of  aqueous  or 
igneous  origin.  Then,  if  it  is  a  sedimentary  rock, 
whether  it  has  been  formed  by  the  action  chiefly  of 
mechanical  forces,  or  of  chemical  and  organic  forces. 
And,  if  it  is  an  eruptive  rock,  whether  it  has  cooled  and 
solidified  below  the  earth's  surface  in  a  fissure,  and  is  a 
dike  or  trappean  rock,  or  has  flowed  out  on  the  sur- 
face and  cooled  in  contact  with  the  air,  and  thus  be- 
come an  ordinary  lava  or  volcanic  rock. 

Here  we  have  the  outlines  of  our  classification,  and 
it  will  be  observed  that  we  have  simply  reached  the 
conclusion,  in  a  somewhat  roundabout  manner,  that 
there  should  always  be  a  general  correspondence  be- 
tween the  classification  of  rocks  and  the  classification 
of  the  forces  that  produce  them.  The  general  plan 
of  the  preceding  scheme  of  the  classification  must 
now  be  clear,  and  the  details  will  be  explained  as  we 
go  along. 


72  STRUCTURAL  GEOLOGY. 

Descriptions  of  Rocks. 

I.  — Sedimentary  or  Stratified  Rocks. 

i.  MECHANICALLY  FORMED  OR  FRAGMENTAL  ROCKS. 
—  These  consist  of  materials  deposited  from  suspension 
in  water,  and  the  process  of  their  formation  is  through- 
out chiefly  mechanical.  The  materials  deposited  are 
mere  fragments  of  older  rocks ;  and,  if  the  fragments 
are  large,  we  call  the  newly  deposited  sediment  gravel ; 
if  finer,  sand ;  and,  if  impalpably  fine,  clay.  These 
fragmental  rocks  cannot  be  classified  chemically,  since 
the  same  handful  of  gravel,  for  instance,  may  contain 
pebbles  of  many  different  kinds  of  rocks,  anjj  thus  be 
of  almost  any  and  very  variable  composition.  Such 
chemical  distinctions  as  can  be  established  are  only 
partial,  and  the  classification,  like  the  origin,  must  be 
mechanical.  Accordingly,  as  just  shown,  we  recognize 
three  principal  groups  based  upon  the  size  of  the  frag- 
ments j  viz. :  — 

(1)  Conglomerate  group. 

(2)  Arenaceous  group. 

(3)  Argillaceous  group. 

This  mode  of  division  is  possible  and  natural,  simply 
because,  as  we  observed  in  an  early  experiment,  mate- 
rials arranged  by  the  -mechanical  action  of  water  are 
always  assorted  according  to  siz«  When  first  depos- 
ited, the  gravel,  sand,  and  clay  are,  of  course,  perfectly 
loose  and  unconsolidated ;  but  in  the  course  of  time 
they  may,  under  the  influence  of  pressure,  heat,  and 
chemical  action,  attain  almost  any  degree  of  consoli- 
dation, becoming  conglomerate,  sandstone,  and  slate, 


LITHOLOGY.  73 

respectively.  The  pressure  may  be  vertical  where  it  is 
due  to  the  weight  of  newer  deposits,  or  horizontal 
where  it  results  from  the  cooling  and  shrinking  of  the 
earth's  interior.  The  heat  may  result  from  mechanical 
movements,  or  contact  with  eruptive  rocks ;  or  it  may 
be  due  simply  to  the  burial  of  the  sediments,  which,  it 
will  be  seen,  must  virtually  bring  them  nearer  the  great 
source  of  heat  in  the  earth's  interior,  on  the  same 
principle  that  the  temperature  of  a  man's  coat,  on  a 
cold  day,  is  raised  by  putting  on  an  overcoat.  The 
effect  of  the  heat,  ordinarily,  is  simply  drying,  cooper- 
ating with  the  pressure  to  expel  the  water  from  the 
sediments ;  but,  if  the  temperature  is  high,  it  may  bake 
or  vitrify  them,  just  as  in  brick-making.  Sediments 
are  consolidated  by  chemical  action  when  mineral 
substances,  especially  calcium  carbonate,  the  iron 
oxides,  and  silica  are  deposited  between  the  particles 
by  infiltrating  waters,  cementing  the  particles  together. 
This  principle  is  easily  demonstrated  experimentally 
by  taking  some  loose  sand  and  wetting  it  repeatedly 
with  a  saturated  solution  of  some  soluble  mineral,  like 
salt  or  alum,  allowing  the  water  to  evaporate  each  time 
before  making  a  fresh  application.  The  interstices 
between  the  grains  are  gradually  filled  up,  and  the 
sand  soon  becomes  a  firm  rock.  But  the  student 
should  clearly  understand  that,  in  geology,  gravel, 
sand,  and  clay  are  just  as  truly  rocks  before  their 
consolidation  as  after.  It  is  plain  then  that  in  each 
of  the  principal  groups  of  fragmental  rocks  we  must 
recognize  an  unconsolidated  division  and  a  consoli- 
dated division. 

(i)    Conglomerate  group.  —  The  rocks  belonging  in 


74  STRUCTURAL    GEOLOGY. 

this  group  we  know  before  consolidation  as  grave}, 
and  after  consolidation  as  conglomerate. 

Gravel.  —  The  pebbles,  as  we  have  already  seen,  are 
usually,  though  not  always,  well  rounded  or  water- 
worn  ;  and  they  may  be  of  any  size  from  coarse  grains 
of  sand  to  boulders.  As  a  rule,  however,  the  larger 
pebbles,  especially,  are  of  approximately  uniform  size 
in  the  same  bed  or  layer  of  gravel,  with,  of  course,  suf- 
ficient fine  material  to  fill  the  interstices.  Although  the 
same  limited  mass  of  gravel  may  show  the  widest  possi- 
ble range  in  chemical  and  mineralogical  composition, 
yet  hard  rocks  are  evidently  more  likely  than  soft  rocks 
to  form  pebbles ;  and  hence  quartz  and  quartz-bearing 
rocks  usually  predominate  in  gravels.  Specimen  28. 

Conglomerate.  —  Consolidated  gravel.  Children 
should  be  led  to  an  appreciation  of  this  point  by  a 
careful  comparison  of  the  forms  of  the  pebbles  in  the 
gravel  and  conglomerate.  The  conglomerate  seems 
to  contain  a  larger  proportion  of  fine  material  than  or- 
dinary gravel.  But  this  is  because  the  gravel  is  usually, 
as  with  our  specimen,  taken  from  the  surface  of  the 
beach,  where,  of  course,  the  pebbles  are  clean  and  sep- 
arate ;  but  if  it  had  remained  there  to  be  covered  by  a 
subsequently  deposited  layer,  enough  fine  stuff  would 
have  been  sifted  into  the  holes  to  fill  them.  And  in 
the  finished  gravel,  just  as  in  the  conglomerate,  the 
pebbles  are  usually  closely  packed,  with  just  sufficient 
sand  and  clay,  or  paste,  as  the  material  in  which  the 
pebbles  are  imbedded  is  called,  to  fill  the  interstices. 
The  paste  is  usually  similar  in  composition  to  the  peb- 
bles, with  this  difference  :  hard  materials  predominate 
in  the  pebbles  and  soft  in  the  paste. 


LITHOLOGY.  75 

Stratified  rocks  generally  show  the  stratification  in 
parallel  lines  or  bands  differing  in  color,  composition, 
etc. ;  but  nothing  like  this  can  be  detected  in  our 
specimens  of  conglomerate ;  and  the  question  might 
be  asked,  How  do  we  know  that  this  is  a  stratified 
rock  ?  In  answer,  it  can  be  said  that  our  hand-speci- 
mens appear  unstratified  simply  because  the  rock  is  so 
coarse ;  but  when  we  look  at  large  masses,  and  espe- 
cially when  we  see  it  in  place  in  the  quarry,  that 
parallel  arrangement  of  the  material  which  we  call  strat- 
ification is  usually  very  evident ;  and  we  often  see  pre- 
cisely the  same  thing  in  gravel  banks.  It  is,  however, 
wholly  unnecessary  that  we  should  see  the  stratification 
in  order  to  know  certainly  that  this  is  a  stratified  or 
aqueous  rock,  because  the  forms  of  the  pebbles  show 
very  plainly  that  they  have  been  fashioned  and  de- 
posited by  moving  water ;  and  we  have  in  the  smallest 
specimen  proof  positive  that  our  conglomerate  is  a 
consolidated  sea-beach. 

Conglomerate  shows  the  same  variations  in  compo- 
sition and  texture  as  gravel ;  it  may  be  composed  of 
almost  any  kind  of  material  in  pebbles  of  almost  any 
size.  We  recognize  two  principal  varieties  of  con- 
glomerate based  on  the  forms  of  the  pebbles ;  if,  as  is 
usual,  these  are  well  rounded  and  water-worn,  the  rock 
is  true  pudding-stone  (specimen  29)  ;  but,  if  they  are 
angular,  or  show  but  little  wear,  it  is  called  breccia. 

(2)  Arenaceous  Group. — The  conglomerate  group 
passes  insensibly  into  the  arenaceous  group  ;  for,  from 
the  coarsest  gravel  to  the  finest  sand,  the  gradation  is 
unbroken,  and  every  sandstone  is  merely  a  conglome- 
rate on  a  small  scale. 


76  STRUCTURAL    GEOLOGY. 

Sand.  —  Like  gravel,  sand  may  be  of  almost  any 
composition,  but  as  a  rule  it  is  quartzose ;  quartz,  on 
account  of  its  hardness  and  the  absence  of  cleavage, 
being  better  adapted  than  any  other  common  mineral 
to  form  sand.  Where  the  composition  of  a  sand  is  not 
specified,  a  quartzose  sand  is  always  understood.  By 
examining  a  typical  sand  with  a  lens,  and  noting  the 
glassy  appearance  of  the  grains,  and  then  testing  their 
hardness  on  a  piece  of  glass,  which  they  will  scratch  as 
easily  as  quartz,  the  pupil  is  readily  convinced  that 
each  grain  is  simply  an  angular  fragment  of  quartz. 
Specimen  30. 

Sandstone.  —  Consolidated  sand.  In  proving  this, 
children  will  notice  first  the  granular  or  sandy  appear- 
ance of  the  sandstone  ;  and  then,  with  the  lens,  that  the 
grains  in  the  sandstone  have  the  same  forms  as  the 
sand-grains.  The  stratification  cannot  be  seen  very 
distinctly  in  our  hand-specimens,  but  in  larger  masses 
it  is  usually  very  plain,  as  may  be  observed  in  the 
blocks  used  for  building,  and  still  better  in  the  quarries. 
However,  even  if  the  stratification  were  not  visible  to 
the  eye,  we  could  have  no  doubt  that  sandstone  is  a 
mechanically  formed  stratified  rock  ;  because  the  form 
of  the  grains,  just  as  in  the  conglomerate,  tells  us  that. 
Many  sandstones,  too,  contain  the  fossil  remains  of 
plants  and  animals,  and  these  are  always  regarded  as 
affording  positive  proof  that  the  rocks  containing  them 
belong  to  the  aqueous  or  stratified  series. 

There  are  many  varieties  of  sandstone  depending 
upon  differences  in  composition,  texture,  etc.,  but  we 
have  not  space  to  notice  them  in  detail.  In  sandstone, 
just  as  in  sand,  quartz  is  the  predominant  constituent, 


LITHOLOGY.  77 

although  we  sometimes  find  varieties  composed  largely 
or  entirely  of  feldspar,  mica,  calcite,  or  other  minerals. 
Specimen  31  is  an  example  of  the  architectural  variety 
known  as  freestone,  which  is  merely  a  fine-grained, 
light-colored,  uniform  sandstone,  not  very  hard,  and 
breaking  with  about  equal  freedom  in  all  directions. 
The  consolidation  of  sandstones  is  due  chiefly  to  chem- 
ical action.  The  cementing  materials  are  commonly 
either  :  ferruginous  (iron  oxides),  giving  red  or  brown 
sandstones  ;  calcareous,  forming  soft  "sandstones,  which 
effervesce  with  acid  if  the  cement  is  abundant;  or 
siliceous,  making  very  strong,  light-colored  sandstones. 
Ferruginous  sandstones  are  the  most  valuable  for  archi- 
tectural purposes  ;  for,  while  not  excessively  hard,  they 
have  a  very  durable  cement.  Siliceous  sandstones  are 
too  hard ;  and  the  calcareous  varieties  crumble  when 
exposed  to  the  weather  because  the  cement  is  soluble 
in  water  containing  carbon  dioxide,  as  all  rain-water 
does.  Specimen  32  is  a  good  example  of  a  ferruginous 
sandstone,  and  it  is  coarse  enough  so  that  we  can  see 
that  each  grain  of  quartz  is  coated  with  the  red  oxide 
of  iron.  The  mica  scales  visible  here  and  there  in  this 
specimen  are  interesting  as  showing  that  the  grains  are 
not  necessarily  all  quartz  ;  and  it  is  important  to  observe 
that  the  mica  was  not  made  in  the  sandstone,  but,  like 
the  quartz,  has  come  from  some  older  rock. 

Quartzite.  —  This  rock  is  simply  an  unusually  hard 
sandstone.  Now  the  hardness  of  any  rock  depends 
upon  two  things :  ( i )  the  hardness  of  the  individual 
grains  or  particles ;  and  (2)  the  firmness  with  which 
they  are  united  one  to  another.  Therefore,  the  hard- 
est sandstones  must  be  those  in  which  grains  of  quarts 


78  STRUCTURAL    GEOLOGY. 

are  combined  with  an  abundant  siliceous  cement ;  and 
that  is  precisely  what  we  have  in  a  typical  quartzite, 
such  as  specimen  33.  Quartzite  is  distinguished,  in 
the  hand-specimen,  from  ordinary  quartz  by  its  granu- 
lar texture  (compare  specimens  15  and  33)  ;  and  of 
course  in  large  masses  the  stratification  is  an  impor- 
tant distinguishing  feature. 

3.  Argillaceous  group.  —  Just  as  the  conglomerate 
group  shades  off  gradually  into  the  arenaceous  group, 
so  we  find  it  difficult  to  draw  any  sharp  line  of  division 
between  the  arenaceous  group  and  the  argillaceous, 
but  we  pass  from  the  largest  pebble  to  the  most  minute 
clay-particle  by  an  insensible  gradation.  For  the  sake 
of  convenience,  however,  we  draw  the  line  at  the  limit 
of  visibility,  and  say  that  in  the  true  clay  and  slate  the 
individual  particles  are  invisible  to  the  naked  eye ;  in 
other  words,  these  rocks  have  a  perfectly  compact  tex- 
ture, while  the  two  preceding  groups  are  characterized 
by  a  granular  texture.  Although  clay,  like  sand  and 
gravel,  may  be  of  almost  any  composition,  yet  it  usually 
consists  chiefly,  often  entirely,  of  the  mineral  kaolin. 
The  reason  for  this  is  easily  found.  Quartz  resists  both 
mechanical  and  chemical  forces,  ana  is  rarely  reduced 
to  an  impalpable  fineness ;  but  all  the  other  common 
minerals,  such  as  feldspar,  hornblende,  mica,  and  cal- 
cite,  on  account  of  their  cleavage  and  inferior  hardness, 
are  easily  pulverized ;  but  it  is  practically  impossible 
that  this  should  happen  without  their  being  broken  up 
chemically  at  the  same  time.  Decomposition  follows 
disintegration ;  and,  while  calcite  is  completely  dis- 
solved and  carried  away,  the  other  minerals  are  reduced, 
as  we  have  seen,  to  impalpable  hydrous  silicates  of 


LITHOLOGY.  79 

aluminum,  /.<?.,  to  kaolin.  Hence,  we  find  that  the 
fragmental  rocks  are  composed  principally  of  two  min- 
erals, quartz  and  kaolin,  —  the  former  predominating  in 
the  conglomerate  and  arenaceous  groups,  and  the  latter 
in  the  argillaceous  group. 

Clay. — That  kaolin  is  the  basis  of  common  clay  is 
proved  by  the  argillaceous  odor,  which  is  so  character- 
istic of  moist  clay.  Pure  kaolin  clay  is  white  and  im- 
palpable, like  China  clay ;  but  pure  clays  are  the  excep- 
tion. They  often  become  coarse  and  gritty  by  admix- 
ture with  sand,  forming  loam;  and  they  also  usually 
contain  more  or  less  carbonaceous  matter,  which  makes 
black  clays ;  or  more  or  less  ferrous  oxide,  which 
makes  blue  clays  ;  or  more  or  less  ferric  oxide,  which 
makes  red,  brown,  and  yellow  clays.  By  mixing  these 
coloring  materials  in  various  proportions,  almost  any 
tint  may  be  explained.  Clays  are  sometimes  calcareous, 
from  the  presence  of  shells  and  shell-fragments  or  of 
pulverized  limestone.  These  usually  effervesce  with 
acid,  and  are  commonly  known  as  marl.  It  is  the 
calcareous  material  in  a  pulverulent  and  easily  soluble 
condition  that  makes  the  marls  valuable  as  soils. 

Slate.  —  Consolidated  clay.  The  compact  texture 
and  argillaceous  odor  are  usually  sufficient  to  prove 
this.  To  get  the  odor  we  need  simply  to  breathe  upon 
the  specimen,  and  then  smell  of  it.  We  find  all 
degrees  of  induration  in  clay.  It  sometimes,  as  every 
one  knows,  becomes  very  hard  by  simple  drying ;  but, 
this  is  not  slate,  and  no  amount  of  mere  drying  will 
change  clay  into  slate  ;  for,  when  moistened  with  water, 
the  dried  clay  is  easily  brought  back  to  the  plastic 
state.  To  make  a  good  slate,  the  induration  must  be 


80  STRUCTURAL    GEOLOGY. 

the  result  of  pressure,  aided  probably  to  some  extent 
by  heat.  True  slate,  then,  is  a  permanently  indurated 
clay  which  will  not  soak  up  and  become  soft  when  wet. 

Slate  is  usually  easily  scratched  with  a  knife,  and  it 
is  distinguished  from  limestone  by  its  non-effervescence 
with  acid.  As  we  should  expect,  it  shows  precisely  the 
same  varieties  in  color  and  composition  as  clay.  A 
good  assortment  of  colors  is  afforded  by  the  roofing- 
slates.  Specimen  34  is  a  typical  slate,  for  it  not  only 
has  a  compact  texture  and  argillaceous  odor,  but  it  is 
very  distinctly  stratified.  The  stratification  is  marked 
by  alternating  bands  of  slightly  different  colors,  and  is 
much  finer  and  more  regular  than  we  usually  observe 
in  sandstone,  and  of  course  entirely  unlike  the  stratifi- 
cation of  conglomerate.  These  differences  are  char- 
acteristic. Some  slates,  however,  are  so  homogeneous 
that  the  stratification  is  scarcely  visible  in  small  pieces. 
Thus  the  roofing-slates  (specimen  35)  rarely  show  the 
stratification ;  for  it  is  an  important  fact  that  the  thin 
layers  into  which  this  variety  splits  are  entirely  inde- 
pendent of  the  stratification.  This  is  the  structure 
known  as  slaty  cleavage  ;  it  is  not  due  to  the  stratifica- 
tion, but  is  developed  in  the  slate  subsequently  to  its 
deposition,  by  pressure.  Some  roofing-slates,  known 
as  ribbon-slates,  show  bands  of  color  across  the  flat  sur- 
faces. These  bands  are  the  true  bedding,  and  indicate 
the  absolute  want  of  conformity  between  this  structure 
and  the  cleavage.  Few  rocks  are  richer  in  fossils  than 
slate,  and  these  prove  that  it  is  a  stratified  rock. 
Slate  which  splits  easily  into  thin  layers  parallel  with 
the  bedding  is  known  as  shale. 

Porcelainite.  —  This  is  clay  or  slate  which  has  been 


LITHOLOGY.  8 1 

baked  or  partially  vitrified  by  heat  so  as  to  have  the 
hardness  and  texture  of  porcelain. 

2 .  CHEMICALLY  AND  ORGANICALLY  FORMED  ROCKS.  — 
We  have  already  learned  that  from  a  geological  point 
of  view  the  differences  between  chemical  and  organic 
deposition  are  not  great,  the  process  being  essentially 
chemical  in  each  case  ;  and  since  the  limestones  and 
some  other  important  rocks  are  deposited  in  both 
ways,  it  is  evidently  not  only  unnatural,  but  frequently 
impossible,  to  separate  the  chemically  from  the  organi- 
cally formed  rocks.  Unlike  the  fragmental  rocks,  the 
rocks  of  this  division  not  only  admit,  but  require,  a 
chemical  classification.  This  is  natural  because  they 
are  of  chemical  origin ;  and  it  is  practicable  because, 
with  few  exceptions,  only  one  class  of  minerals  is  de- 
posited at  the  same  time  in  the  same  place,  —  a  very 
convenient  and  important  fact.  Therefore  our  arrange- 
ment will  be  mineralogical,  thus  :  — 

(1)  Coal  Group. 

(2)  Iron-ore  Group. 

(3)  Siliceous  Group. 

(4)  Calcareous  Group. 

(5)  Metamorphic  Group  (Silicates). 

Most  of  the  silicate  rocks  are  mixed,  *>.,  are  each 
composed  of  several  minerals ;  but  some  silicate  rocks 
and  all  the  rocks  of  the  other  divisions  are  simple,  each 
species  consisting  of  a  single  mineral  only. 

( i )  Coal  Group.  —  These  are  entirely  of  organic 
origin,  and  include  two  allied  series,  which  are  always 
merely  the  more  or  less  extensively  transformed  tissues 
of  plants  or  animals ;  viz. :  — 


82  STRUCTURAL    GEOLOGY. 

Coals  and  Bitumens.  —  At  the  first  lesson  we  exam- 
ined a  sample  of  peat  (specimen  8),  and  considered 
the  general  conditions  of  its  formation,  peat  being  in 
every  instance  simply  partially  decayed  marsh  vegeta- 
tion. It  was  also  stated  that,  as  during  the  lapse  of 
time  the  transformation  becomes  more  complete,  the 
peat  is  changed  in  succession  to  lignite,  bituminous 
coal,  anthracite,  and  graphite.  The  coals,  indeed, 
make  a  very  beautiful  and  perfect  series,  whether  we 
consider  the  composition — there  being  a  gradual,  pro- 
gressive change  from  the  composition  of  ordinary 
woody  fibre  in  the  newest  peat  to  the  pure  carbon  in 
graphite,  —  or  the  degree  of  consolidation  and  mineral- 
ization—  since  there  is  a  gradual  passage  from  the  light, 
porous  peat,  showing  distinctly  the  vegetable  forms,  to 
the  heavy  crystalline  graphite,  bearing  no  trace  of  its 
vegetable  origin.  This  relation  is  easily  appreciated 
by  a  child,  if  a  proper  series  of  specimens  is  presented. 
The  coals  also  make  a  chronological  series,  graphite 
and  anthracite  occurring  only  in  the  older  formations, 
and  lignite  and  peat  in  the  newer,  while  bituminous 
coal  is  found  in  formations  of  intermediate  age. 

Bituminous  coal  is  the  typical,  the  representative 
coal;  and  from  a  good  specimen  of  this  variety  we 
may  learn  two  important  facts  :  — 

(1)  That  true  coals,  no  less  than  peat,  are  of  vege- 
table origin.     To  see  this  we  must  look  at  the  flat  or 
charcoal  surfaces  of  the  coal.     These  soil  the  fingers 
like  charcoal,  and  usually  show  the  vegetable  forms 
distinctly. 

(2)  That  coals  are  stratified  rocks.     These  dirty 
charcoal  surfaces  always  coincide  with  the  stratification, 


LITHOLOGY.  83 

being  merely  the  successive  layers  of  vegetation  de- 
posited and  pressed  together  to  build  up  the  coal ;  and 
when  we  look  at  the  edge  of  the  specimen  the  stratifi- 
cation shows  plainly  enough. 

The  bitumens  form  a  similar  though  less  perfect 
series,  beginning  with  the  organic  tissues,  and  ending, 
in  the  opinion  of  some  of  the  best  chemists  and  miner- 
alogists, with  diamond.  In  fact  the  coals  and  bitumens 
form  two  distinct  but  parallel  series.  The  coals  are 
exclusively  of  vegetable  origin,  while  the  bitumens  are 
largely  of  animal  origin.  The  organic  tissues  in  which 
the  two  series  originate  are  chemically  similar,  —  the 
animal  tissues,  which  produce  the  lighter  forms  of  bitu- 
men, however,  containing  more  hydrogen  and  less 
carbon  and  oxygen  than  vegetable  tissues ;  while  the 
final  terms,  as  just  shown,  are  probably  chemically 
identical,  being  pure  carbon,  —  graphite  for  the  coals 
and  diamond  for  the  bitumens  ;  so  that  the  entire  pro- 
cess of  change  in  each  series  is  essentially  carboniza- 
tion, a  gradual  elimination  of  the  gaseous  elements, 
oxygen  and  hydrogen,  until  pure  solid  carbon  alone 
remains. 

The  principal  differences  between  the  coals  and 
bitumens  are  the  following  :  — 

Coals  are  rich  in  carbon,  with  some  oxygen  and  little  hydrogen. 
Bitumens  are  rich  in  hydrogen,  with  some  carbon  and  little  or 

no  oxygen. 

Coals  are  entirely  insoluble. 
Bitumens  are  soluble  in  ether,  benzole,  turpentine,  etc.,  and  the 

solid  forms  are  soluble  in  the  more  fluid,  naphtha-like  varieties. 
Coals  are  never  liquid,  and  cannot  be  melted  or,  with  trifling 

exceptions,  even  softened  by  heat. 
Many  bitumens  are  naturally  liquid,  and  all  become  so  on  the 

application  of  heat. 


»4  STRUCTURAL    GEOLOGY. 

The  coals  partake  of  the  characteristics  of  their  chief  constituent 
element,  carbon,  the  most  thoroughly  solid  substance  known ; 
while  the  bitumens  similarly  show  the  influence  of  hydrogen, 
the  most  perfectly  fluid  substance  known. 

The  two  bitumens  of  the  greatest  geological  impor- 
tance are  asphaltum  or  mineral  pitch  and  petroleum ; 
but  these  substances  are  too  familiar  to  require  any 
farther  description  here. 

(2)  Iron-ore  Group.  —  These  interesting  and  im- 
portant stratified  rocks  include  the  three  principal 
oxides  of  iron,  —  limonite,  hematite,  and  magnetite, 
—  as  well  as  the  carbonate  cf  iron,  siderite ;  and  the 
rocks  have  essentially  the  same  characteristics  as  the 
minerals.  In  economical  importance  they  are  second 
only  to  the  coals ;  and  the  history  of  their  formation 
through  the  agency  of  organic  matter  is  one  of  the 
most  interesting  chapters  in  chemical  geology  (see 
page  26).  The  three  oxides  are  easily  distinguished 
from  each  other  by  the  colors  of  their  powders  or 
streaks,  and  the  magnetism  of  magnetite,  and  from 
all  other  common  rocks  by  their  high  specific  gravity. 
Magnetite  is  the  richest  in  iron,  and  limonite  the 
poorest.  As  regards  the  degree  of  crystallization 
and  order  of  occurrence  in  the  formations,  they  form 
a  series  parallel  with  the  coal  series,  thus  :  — 

Limonite,  never  crystalline,  and  found  in  recent  formations. 
Hematite,  often  crystalline,  and  found  in  older  formations. 
Magnetite,  always  crystalline,  and  found  in  oldest  formations, 

Siderite  effervesces  with  strong  acid ;  and  this  sepa- 
rates it  from  all  other  rocks,  except  limestone  and 
dolomite ;  and  from  these  it  is  distinguished  by  its 


LITHOLOGY.  85 

high  specific  gravity.  As  a  mineral,  siderite  is  often 
light  colored ;  but  as  a  rock  it  is  always  dark,  and 
usually  black,  from  admixture  chiefly  of  carbonaceous 
matter.  In  studying  dynamical  geology,  we  have 
learned  (page  28)  tru;  reason  for  the  intimate  asso- 
ciation of  siderite  with  beds  of  coal,  and  this  accounts 
equally  for  the  carbon  contained  in  the  rock  itself. 
The  connection  of  this  rock  with  the  coal-formations 
adds  much  to  its  value  as  an  ore  of  iron. 

Finally,  the  iron-ores,  at  least  where  of  much  eco- 
nomical importance,  are  truly  stratified.  This  can 
often  t>e  seen  in  hand-specimens ;  and  is  well  shown 
by  their  relations  to  other  rocks,  in  quarries  and 
mines ;  and  in  many  cases,  for  limonite  and  hema- 
tite, by  the  fossils  which  they  contain. 

(3)  Siliceous  Group.  —  These  rocks  are  composed 
of  pure  silica  in  the  forms  of  quartz  and  opal.  When 
first  deposited,  whether  organically,  like  tripolite,  or 
chemically,  like  siliceous  tufa,  the  siliceous  rocks  are 
soft  and  light,  and  the  silica  is  in  the  form  of  opal. 
Subsequently  it  changes  to  quartz,  and  the  rocks  as- 
sume the  much  harder  and  denser  forms  of  chert  and 
novaculite,  respectively. 

Tripolite  or  Diatomaceous  Earth.  —  This  interest- 
ing rock  is  soft,  light,  and  looks  like  clay ;  but  it  is 
lighter,  and  the  argillaceous  odor  is  faint  or  wanting. 
It  does  not  effervesce  with  acid.  Hence,  it  is  neither 
clay  nor  chalk.  Notwithstanding  its  softness,  it  is  really 
composed  of  a  hard  substance,  viz.,  silica,  in  the  form 
known  as  opal.  By  rubbing  off  a  little  of  the  dust,  and 
examining  it-  under  the  microscope,  we  easily  prove 
that  the  silica  is  mainly  or  entirely  of  organic  origin ; 


86  STRUCTURAL    GEOLOGY. 

for  the  dust  is  seen,  to  be  simply  a  mass  of  more  or  less 
fragmentary  organic  remains,  occurring  in  great  variety, 
and  of  wonderful  beauty  and  minuteness.  There  are 
few  rocks  so  unpromising  on  the  exterior,  and  yet  so 
beautiful  within.  We  have  already  learned  that  these 
organic  bodies  are  principally  Diatom  cases,  Radiolaria 
shells,  and  Sponge  spicules.  We  can  form  some  idea 
of  their  minuteness  from  Ehrenberg's  estimate  that  a 
single  cubic  inch  of  pure  tripolite  contained  no  less 
than  41,000,000,000  organisms. 

The  lightness  of  tripolite  (sp.  gr.,  1-1.5)  *s  due  to 
the  facts  that  opal  is  a  light  mineral  (sp.  gr.,  1.9-2.2), 
and  that  many  of  the  shells  are  hollow.  Tripolite  is  a 
good  example  of  a  soft  rock  composed  of  a  hard  min- 
eral ;  and  it  owes  its  value  as  a  polishing  material  to 
the  fact  that  it  consists  of  a  hard  mineral  in  an  exceed- 
ingly fine  state  of  division.  Tripolite,  when  pure,  is 
snow-white ;  but  it  is  rarely  pure,  being  commonly 
either  argillaceous  or  calcareous.  This  rock  is  now 
forming  in  thousands  of  places,  in  both  fresh  water 
and  the  ocean. 

Flint  and  Chert. —  During  the  course  of  geological 
time,  beds  of  tripolite  are  gradually  consolidated,  chiefly 
by  percolating  waters,  which  are  constantly  dissolving 
and  re-depositing  the  silica ;  and,  finally,  in  the  place 
of  a  soft,  earthy  rock,  we  get  a  hard,  flinty  one,  whicn 
we  call  flint  if  it  occurs  in  the  newer,  or  chert  if  it 
occurs  in  the  older,  geological  formations.  Besides 
forming  beds  of  nearly  pure  silica,  which  we  call  tripo- 
lite, the  microscopic  siliceous  organisms  are  diffused 
more  or  less  abundantly  through  other  rocks,  especially 
chalk  and  limestone.  In  such  cases  the  consolidatiois 


LITHOLOGY.  87 

of  the  silica  implies  its  segregation  also ;  /.<?.,  the  silica 
dissolved  by  percolating  water  is  deposited  only  about 
certain  points  in  the  rock,  building  up  rounded  con- 
cretions or  nodules.  Thus,  a  siliceous  limestone  be- 
comes, by  the  segregation  of  the  silica,  a  pure  limestone 
containing  nodules  of  chert,  which  are  usually  arranged 
in  lines  parallel  with  the  stratification.  Specimen  16 
is  a  fragment  of  a  flint-nodule  from  the  chalk-forma- 
tion of  England. 

Siliceous  Tufa.  —  Hot  water,  and  especially  hot  alka- 
line water,  circulating  through  the  earth's  crust,  is 
always  charged  with  silica  dissolved  out  of  the  rocks ; 
and  when  such  water  issues  on  the  surface  in  a  hot 
spring  or  geyser,  it  is  cooled  by  contact  with  the  air, 
its  solvent  power  is  diminished  thereby,  and  a  large 
part  of  the  silica  is  deposited  around  the  outlet  as  a 
snowy-white  porous  material  called  siliceous  tufa.  Sil- 
ica deposited  in  this  way  is,  like  organic  silica,  always 
in  the  form  of  opal.  Siliceous  tufa  is  distinguished 
from  clay,  slate,  chalk,  and  limestone  by  the  same 
tests  as  tripolite,  and  from  tripolite  itself  by  the  ab- 
sence of  microscopic  organisms. 

Novaculite.  —  Through  the  action  of  percolating 
water  and  pressure,  siliceous  tufa,  like  tripolite,  be- 
comes harder  and  denser  and  is  thus  changed  to 
novaculite,  which  holds  the  same  relation  to  chemi- 
cally deposited  silica  that  chert  and  flint  do  to  organi- 
cally deposited  silica.  The  white  novaculite  obtained 
at  the  Hot  Springs  of  Arkansas,  and  commonly  known 
as  Arkansas  stone,  is  a  typical  example  of  this  rock. 
The  rock  which,  on  account  of  the  use  to  which  it  is 
put,  is  known  as  buhr- stone,  is  also  an  excellent  exam- 


88  STRUCTURAL    GEOLOGY. 

pie  of  chemically  deposited  silica.  It  is  usually  some- 
what porous  and  fossiliferous. 

(4)  Calcareous  Group.  —  These  are  the  lime-rocks; 
including  the  carbonate  of  lime,  in  limestone  and  dolo- 
mite, the  sulphate  of  lime,  in  gypsum,  and  the  phos- 
phate of  lime,  in  phosphate  rock.  These  rocks  are 
even  more  closely  connected  in  origin  than  in  compo- 
sition ;  and  it  is  for  this  reason  that  rock-salt,  which 
of  course  contains  no  lime,  is  also  included  in  this 
group.  Limestones  are  formed  abundantly  in  the 
open  sea,  through  the  accumulation  of  shells  and 
corals;  but  when  portions  of  the  sea  become  de- 
tached from  the  main  body  and  gradually  dry  up, 
like  the  Dead  Sea  and  Great  Salt  Lake,  dolomite, 
gypsum,  and  rock-salt  are  deposited  in  succession  as 
chemical  precipitates.  Phosphate  rock  may  be  re- 
garded as  a  variety  of  limestone,  resulting  from  the 
accumulation  of  the  skeletons  and  excrement  of  the 
higher  animals. 

Limestone.  —  This  is  the  lithologic  or  rock  form 
of  carbonate  of  lime  or  calcite,  and  one  of  the 
most  important,  interesting,  and  useful  of  all  rocks. 
Although  so  simple  in  composition,  —  calcite  bemg 
the  only  essential  constituent,  —  limestone  embraces 
many  distinct  varieties,  and  is  really  equivalent  to  a 
whole  family  of  rocks.  A  highly  fossiliferous  lime- 
stone, such  as  specimen  38,  is,  perhaps,  the  best 
variety  with  which  to  begin  the  study  of  the  species. 
The  softness  of  the  fossil  shells  of  which  the  rock  is 
so  largely  composed,  and  the  fact  that  they  effervesce 
readily  with  dilute  acid,  proves  that  they  are  still  car- 
bonate of  lime ;  and  by  applying  the  acid  more  care- 


LIT  HO  LOGY.  89 

fully,  it  can  be  seen  that  the  compact  matrix  of  the 
rock  also  effervesces,  consisting  of  shells  more  finely 
broken  or  comminuted  and  mixed  with  more  or  less 
clay  and  other  impurity,  almost  the  entire  rock  being 
of  organic  origin. 

On  the  coast  of  Florida,  and  in  many  other  places,  we 
find  beautiful  examples  of  shell-limestone  now  in  pro- 
cess of  formation.  These  are  at  first  very  open  and 
porous,  because  the  interstices  between  the  nearly  entire 
shells  are  not  yet  filled  up  with  smaller  fragments  and 
sand.  But  when  that  is  done,  we  shall  have  a  rock 
similar  to  the  old  fossiliferous  limestone.  Specimen  37. 

The  shells  and  fragments,  and  the  grains  of  calca- 
reous sand,  are,  as  a  rule,  quickly  cemented  together 
by  the  deposition  of  carbonate  of  lime  between  them  ; 
so  that  limestone  is  nowhere  observed  occurring  abun- 
dantly in  an  unconsolidated  form. 

Limestone,  as  a  rule,  is  not  distinctly  stratified  in 
hand-specimens,  but  of  course  that  it  is  a  true  sedi- 
mentary rock  is  abundantly  proved  by  the  fossils; 
and  it  goes  almost  without  saying  that  limestone,  be- 
ing necessarily  mainly  composed  of  organic  remains, 
must  be  to  a  greater  extent  than  any  other  rock  the 
great  store-house  of  fossils ;  and  in  no  other  rock  are 
the  fossils  so  well  preserved  and  perfect  as  in  limestone. 

Nevertheless,  there  are  extensive  formations  of  lime- 
stone containing  no  discernible  traces  of  fossils.  And 
some  of  these  non-fossiliferous  limestones,  too,  are  of 
very  recent  formation.  Some  of  the  modern  coral- 
reefs,  for  example,  are  composed  of  limestone  which 
was  formed  only  yesterday,  as  it  were,  and  which,  from 
its  mode  of  formation,  must  consist  entirely  of  corals ; 


90  STRUCTURAL    GEOLOGY. 

and  yet  it  shows  no  trace  of  its  organic  origin,  but  is 
perfectly  compact,  or,  possibly,  crystalline.  This  fre- 
quent obliteration  of  the  organic  remains,  as  well  as 
the  perfect  consolidation  of  the  rock,  is  attributed  to 
its  solubility.  The  calcium  carbonate  is  gradually  dis- 
solved by  the  water,  and  then  deposited  in  the  inter- 
stices in  other  parts  of  the  rock. 

Specimen  39  is  that  variety  of  limestone  known  as 
chalk.  It  is  soft  and  earthy,  resembling  both  clay  and 
tripolite,  but  differing  from  the  former  in  lacking  the 
distinct  argillaceous  odor,  and  from  both  by  its  lively 
effervescence  with  acids.  It  appears  to  be  entirely  des- 
titute of  organic  remains,  but  this  is  a  defect  of  our  vis- 
ion and  not  of  the  rock ;  for,  like  the  tripolite,  it  often 
appears  under  the  microscope  to  be  little  else  than  a 
mass  of  shells.  Tripolite  is  a  deposit  built  up  of  the 
siliceous  shells  of  Diatoms  and  Radiolaria,  while  chalk 
is  chiefly  composed  of  the  similar  but  calcareous  shells 
of  Foraminifera.  Our  specimen  is  from  the  Cretaceous 
formation  of  England ;  but  we  have  good  reason  to 
believe  that  chalk  is  now  forming  on  a  very  extensive 
scale.  There  are  millions  of  square  miles  in  the  deeper 
parts  of  the  ocean  where  the  dredge  brings  up  little 
else  but  a  perfectly  impalpable,  gray,  calcareous  slime 
or  ooze.  When  examined  microscopically,  this  is  seen 
to  be  composed  chiefly  of  Foraminifera  shells,  and 
among  these  the  genus  Globigerina  predominates ;  so 
that  the  deposit  is  frequently  called  Globigerina  ooze. 
Now  this  gray,  calcareous  ooze,  when  dried  and  com- 
pacted by  pressure,  makes  a  soft,  white  rock  which 
can  scarcely  be  distinguished  from  chalk ;  in  fact,  it  is 
a  modern  chalk.  And  there  seems  no  good  reason 


LITHOLOGY.  91 

to  doubt  that  the  deposition  of  chalk  has  gone  on  con- 
tinuously since  Cretaceous  time  —  for  several  millions 
of  years  at  least. 

Specimen  40  is  also  a  white  rock,  easily  scratched 
with  the  knife,  and  effervescing  freely  with  acid,  and 
therefore  a  variety  of  limestone.  But  its  texture  is  very 
different  from  the  other  varieties  we  have  studied.  It 
has  a  sparkling  surface,  which  we  explain  by  saying  that 
the  rock  is  crystalline.  It  is,  in  fact,  a  mass  of  minute 
crystals  of  calcite.  The  crystalline  limestones  have 
not  always  been  crystalline,  but  it  is  safe  to  assume 
that  they  were  originally  entirely  uncrystalline,  and  in 
many  cases  rich  in  fossils ;  but  the  fossils  have  been 
mainly  obliterated  by  the  crystallization. 

Crystallization  generally  in  rocks  is  an  indication  of 
great  age,  so  that  we  usually  say  crystalline  rocks 
must  be  older  than  uncrystalline  rocks  of  the  same 
composition;  and  this  is  mainly  true  with  the  lime- 
stones. When  the  crystallization  is  rather  fine,  as  in 
our  specimen,  resembling  granulated  sugar,  we  have 
what  is  commonly  called  saccharoidal  limestone.  This 
is  the  typical  marble.  Marble  is  not  a  scientific  name, 
and  the  term  is  usually  applied  to  any  calcareous  rock 
which  will  take  a  polish,  and  sometimes  even  to  rocks 
which  are  not  calcareous  at  all. 

In  the  section  on  dynamical  geology,  we  learned  that 
the  carbonate  of  calcium  or  calcite  is  deposited  from 
the  sea-water,  and  limestones  formed,  in  two  ways  :  first, 
in  a  purely  chemical  way,  where  the  water  becomes 
saturated  with  calcite  ;  and,  second,  organically,  where 
the  calcium  carbonate  is  taken  from  the  water  by  ma- 
rine organisms  to  form  their  shells  and  skeletons,  and 


92  STRUCTURAL    GEOLOGY. 

the  gradual  accumulation  of  these  on  the  ocean-floor 
builds  up  a  limestone.  As  before  stated,  the  difference 
between  these  two  methods  of  deposition  is  not  so 
great  as  it  often  seems,  because  we  know  that  the  ani- 
mals never  make  the  carbonate  of  calcium  which  they 
secrete,  but  it  comes  into  the  sea  ready  made  with  the 
drainage  from  the  land. 

The  limestones  forming  at  the  present  time  are 
almost  wholly  organic ;  but  the  rock  known  as  cal- 
careous tufa  is  an  exception.  This  is  formed  under 
the  same  general  conditions  as  siliceous  tufa,  but  much 
more  abundantly,  and  in  cold  water  as  well  as  warm  ; 
because  calcite  is  far  more  soluble  (especially  in  water 
containing  carbon  dioxide)  than  opal  or  quartz.  It  is 
deposited,  not  only  around  the  mouths  of  springs,  but 
also  along  the  beds  of  the  streams  which  they  form, 
enveloping  stones,  roots,  grasses,  etc.,  and  building  up 
usually  a  loose,  spongy  mass  having  a  very  character- 
istic turfaceous  texture. 

The  principal  accessory  minerals  occurring  in  lime- 
stone are :  ( i )  kaolin,  forming  argillaceous  or  slaty  lime- 
stone, which  may  be  recognized  by  the  argillaceous 
odor  and  dark  color;  (2)  quartz,  forming  siliceous 
or  cherty  limestone,  known  by  its  hardness  or  by  the 
nodules  of  flint  or  chert;  (3)  dolomite,  forming  dolo- 
mitic  or  magnesian  limestone,  which  effervesces  less 
freely  with  acid;  and  (4)  serpentine,  forming  serpen- 
tinic  limestone,  which  is  sharply  distinguished  by  the 
green  grains  of  serpentine  mingled  with  the  white  cal- 
cite. A  concretionary  texture  is  common  with  lime- 
stone. If  the  concretions  are  small,  like  mustard-seed, 
we  call  the  rock  oolite  ;  if  larger,  like  peas,  pisolite. 


LITHOLOGY.  93 

Dolomite.  —  If  for  calcite,  which  is  the  sole  essential 
constituent  of  all  limestone,  we  substitute  the  allied 
mineral  dolomite,  we  have  the  rock  dolomite.  As 
might  be  inferred  from  its  composition,  dolomite  is 
very  closely  related  to  limestone,  although  there  are 
some  important  differences.  Physically,  the  two  rocks 
differ  about  as  the  two  minerals  do.  Dolomite  is 
harder  than  limestone,  and  being  also  less  soluble,  it 
resists  the  action  of  the  weather  more.  Dolomite,  if 
pure,  effervesces  feebly,  or  not  at  all,  with  cold  dilute 
acid.  Here,  however,  we  have  to  recognize  the  fact 
that  dolomite  is  rarely  pure  ;  but  there  exists,  in  conse- 
quence of  the  admixture  of  calcite,  a  perfectly  gradual 
passage  from  pure  dolomite  to  pure  limestone,  and 
parallel  with  this  every  degree  of  vigor  in  the  reaction 
with  acid.  Hence,  it  is  entirely  an  arbitrary  matter 
as  to  where  we  shall  draw  the  line  between  dolomitic 
limestone  and  calcareous  dolomite.  Dolomite  is  a  very 
much  less  abundant  rock  than  limestone,  and,  unlike 
limestone,  it  rarely  contains  many  fossils,  and  is  never 
of  organic  origin ;  i.e.,  there  are  no  organisms  which 
secrete  the  mineral  dolomite  to  form  their  hard  parts 
or  skeletons.  Like  gypsum  and  rock-salt,  dolomite 
is  probably  nevei  deposited  in  the  open  ocean,  but 
only  in  closed  basins.  Like  limestone,  it  occurs  with 
both  the  compact  and  the  crystalline  textures. 

Gypsum.  —  When  pure,  this  rock  (specimen  36)  is 
identical  with  the  mineral  gypsum  (specimen  17),  ex- 
cept that  it  is  rarely  crystalline.  It  is  usually,  however, 
not  only  perfectly  compact,  but  more  or  less  dark- 
colored  from  the  admixture  of  clay  and  other  impuri- 
ties. Its  most  notable  characteristics  are  its  softness,  the 


94  STRUCTURAL    GEOLOGY. 

absence  of  the  argillaceous  odor,  except  where  it  con- 
tains much  clayey  impurity,  and  its  non-effervescence 
with  acids.  The  first  two  usually  serve  to  distinguish 
it  from  slate,  while  the  acid  test  separates  it  readily 
from  limestone  and  all  other  carbonate  rocks.  The 
deposition  of  gypsum  is  purely  chemical,  and  it  occurs 
under  about  the  same  physical  conditions  as  the  depo- 
sition of  salt ;  i.e.,  in  drying-up  portions  of  the  sea. 
Hence  we  usually  find  gypsum  associated  with  beds 
of  rock-salt ;  and,  since  drying-up  seas  are  few  in 
number,  and  small  compared  with  the  whole  extent 
of  the  ocean,  we  can  easily  understand  why  neither 
rock-salt  nor  gypsum  are  abundant  rocks,  except  in 
a  few  localities. 

Rock- Salt.  —  This  interesting  and  useful  rock,  as  we 
have  already  learned,  is  deposited  in  a  purely  chemical 
way,  and  only  in  drying-up  portions  of  the  sea,  like  the 
Dead  Sea,  Great  Salt  Lake,  etc.  In  some  parts  of 
Europe  there  are  beds  of  solid  rock-salt  over  a  hun- 
dred feet  thick. 

Phosphate  Rock.  —  Although  not  specially  abundant 
or  attractive,  this  rock  is  of  great  economic  interest 
and  importance  on  account  of  its  extensive  use  as  a 
fertilizer.  Under  the  general  head  of  phosphate  rock 
are  included  :  ( i )  the  typical  guano,  which  is  the 
consolidated  excrement  of  certain  marine  birds  in- 
habiting in  great  numbers  small  coral  islands  in  the 
dry  or  rainless  regions  of  the  tropics ;  (2)  the  under- 
lying coral  rock,  which  is  often  changed  to  phosphate 
rock  through  the  percolation  of  the  rain-water  falling 
on  the  guano ;  (3)  accumulations  of  the  bones  and 
coprolites  of  the  higher  animals  ;  (4)  phosphatic  lime- 


LITHOLOGY.      '-  95 

Stones  from  which  the  carbonate  of  lime  has  been 
Jargely  dissolved  away,  leaving  the  more  insoluble 
phosphate  of  lime. 

(5)  Metamorphic  Group  (stratified  silicates).— 
All  the  chemically  and  organically  formed  rocks  which 
we  have  studied  up  to  this  point  are  simple,  i.e., 
they  consist  each  of  only  one  essential  mineral ;  but 
most  of  the  rocks  in  this  great  group  of  silicates  are 
mixed,  or  consist  each  of  several  essential  minerals. 
Quartz  is  the  only  important  constituent  of  these  rocks 
which  is  not,  strictly  speaking,  a  silicate,  but  in  a  cer- 
tain sense  it  is  also  not  an  exception,  since  it  may 
always  be  regarded  as  an  excess  of  acid  in  the  rock. 

This  group  of  stratified  rocks  composed  of  silicate 
minerals  is  of  exceptional  importance,  first,  on  account 
of  the  large  number  of  species  which  it  includes,  and, 
second,  on  account  of  the  vast  abundance  of  some  of 
the  species.  These  are,  above  all  others,  the  rocks  of 
which  the  earth's  crust  is  composed.  With  unimpor- 
tant exceptions,  all  the  rocks  of  this  group  are  crystal- 
line ;  and  they  constitute  the  principal  part  of  what  is 
generally  included  under  the  term  metamorphic  rocks  — 
a  general  name  for  all  stratified  rocks  which  have  been 
so  acted  upon  by  heat,  pressure,  or  chemical  forces  as 
to  make  them  crystalline.  Although  the  crystalline 
limestone,  dolomite,  iron-ores,  etc.,  show  us  that  meta- 
morphic rocks  are  not  wanting  in  the  other  groups. 

As  already  explained,  the  metamorphic  or  crystalline 
stratified  rocks  are  usually  older  than  the  corresponding 
uncrystalline  rocks  ;  but  a  point  of  greater  importance 
here  is  this  :  the  development  in  the  silicate  rocks  of 
crystalline  characters  has  usually  made  it  impossible  to 


96  STRUCTURAL    GEOLOGY. 

determine  the  method  of  their  deposition,  whether 
mechanical  or  chemical.  In  a  few  cases,  as  with  the 
rock  greensand,  we  know  that  the  deposition  is  chemi- 
cal; while  it  is  equally  certain  that  such  common 
silicate  rocks  as  gneiss,  mica  schist,  and  many  others, 
often  result  from  the  crystallization  of  ordinary  me- 
chanical sediments,  like  sandstone  and  conglomerate. 
We  classify  all  these  rocks  as  of  chemical  origin,  how- 
ever, without  considering  the  mode  of  their  deposition, 
because  the  subsequent  crystallization  is  itself  essen- 
tially a  chemical  process  ;  and  that  justifies  us  in  saying 
that  these  rocks  are  made  what  they  now  are  chiefly 
by  the  action  of  chemical  forces.  Whatever  they  were 
originally,  they  have  become,  through  their  crystalli- 
zation, rocks  having  a  definite  mineral  composition 
which  can  be  classified  chemically. 

Some  of  the  details  of  the  classification  of  this  group, 
as  shown  in  the  table,  require  explanation.  In  study- 
ing the  silicate  minerals  it  was  stated  to  be  important 
to  recognize  two  classes  —  the  acidic  and  the  basic — 
the  dividing  line  falling  in  the  neighborhood  of  60  per 
cent  of  silica.  This  division  is  important  simply  be- 
cause Nature  has  in  a  great  degree  kept  the  acidic  and 
basic  minerals  separate  in  the  rocks ;  and  few  things  in 
lithology  are  more  important  than  the  distinction  of  the 
silicate  rocks  in  which  acidic  minerals  predominate 
from  those  in  which  basic  minerals  predominate.  The 
amount  of  silica  which  any  rock  of  this  group  contains 
is  shown  at  a  glance  by  the  chart.  The  vertical  broken 
lines,  with  the  figures  at  the  top,  indicate  the  propor- 
tion of  silica,  which  increases  from  30  per  cent  on  the 
right  to  85  per  cent  on  the  left ;  so  that  the  percentage 


LTTHOLOGY.  97 

of  silica  which  a  rock  contains  determines  its  position, 
the  acidic  species  being  on  the  left,  and  the  basic  on 
the  right.  As  most  of  these  rocks  are  composed  of 
two  or  more  minerals  mixed  in  very  various  proportions, 
there  is  usually  a  wide  range  in  the  percentage  of  silica 
which  the  same  species  may  contain ;  and  this  is  ex- 
pressed in  each  case  by  the  length  of  the  dotted  line 
under  the  name  of  the  rock.  Thus,  in  syenite,  the  sil- 
ica ranges  from  55  per  cent,  to  65  per  cent.  The  hori- 
zontal line  in  the  chart  separates  the  gneisses,  containing 
feldspar  as  an  essential  constituent,  from  the  schists,  in 
which  feldspar  is  wanting,  except  as  an  accessory  con- 
stituent. We  will  take  up  the  gneisses  first. 

Gneiss.  —  This  is  the  most  important  of  all  rocks. 
It  forms  not  far  from  one-half  of  New  England,  and  a 
very  large  proportion  of  the  earth's  crust  The  name 
(pronounced  same  as  nice)  is  known  to  have  origi- 
nated among  the  Saxon  miners,  but  its  precise  deriva- 
tion is  lost  in  obscurity.  To  find  out  what  this  very 
important  rock  is,  we  will  consult  specimen  41.  The 
first  glance  shows  us  that  it  is  not,  like  the  rocks  we 
have  just  been  studying,  composed  of  a  single  mineral, 
but  of  several  minerals,  the  most  conspicuous  of  which 
is  the  pink  feldspar  —  orthoclase.  This  we  recognize 
as  a  feldspar  :  (i)  by  its  hardness,  which  is  a  little  less 
than  that  of  quartz,  and  distinguishes  it  from  calcite, 
a  mineral  having  the  general  appearance  of  feldspar ; 
(2)  by  its  color,  which  separates  it  from  hornblende 
and  augite  ;  and  (3)  by  its  cleavage,  which  distinguishes 
it  easily  from  quartz.  Finally,  we  know  it  is  orthoclase, 
and  not  plagioclase,  by  its  general  aspect,  and  by  its 
association  with  an  abundance  of  quartz,  which  is  the 


98  STRUCTURAL    GEOLOGY. 

next  most  important  constituent  of  the  rock.  The 
quartz  is  less  abundant  than  the  orthoclase,  and  more 
easily  overlooked,  yet  anyone  familiar  with  the  min- 
eral will  not  fail  to  recognize  it.  It  forms  small, 
irregular,  glassy  grains,  entirely  devoid  of  cleavage,  and 
scratching  glass  easily.  On  weathered  surfaces  of  the 
rock  the  orthoclase  becomes  soft  and  chalky,  while  the 
quartz  remains  clear  and  hard,  and  then  the  two  min- 
erals are  very  easily  distinguished.  Besides  these, 
there  are  numerous  black,  thin,  glistening  scales,  which 
we  can  easily  prove  to  be  elastic,  and  recognize  as 
mica. 

In  most  books  on  the  subject,  these  three  minerals 
—  orthoclase,  quartz,  and  mica — are  set  down  as  the 
normal  or  essential  constituents  of  gneiss.  But  it  is 
now  recognized  by  the  best  lithologists  that  we  may 
have  true  gneiss  without  any  mica;  or  we  may  have 
hornblende  in  the  place  of  mica.  Quartz  and  ortho- 
clase are  the  only  essential  constituents  of  gneiss  ;  and 
when  these  alone  are  present,  we  have  the  variety 
known  as  binary  gneiss.  The  addition  to  these  essen- 
tial constituents  of  mica,  gives  micaceous  gneiss  ;  and 
of  hornblende,  hornblendic  gneiss.  Of  these  three 
principal  varieties,  the  micaceous  gneiss  is  by  far  the 
most  common  and  important.  The  mica  may  be 
either  the  white  species,  muscovite,  or  the  black  spe- 
cies, biotite ;  but  it  is  usually  the  former. 

Orthoclase  is  the  predominant  constituent  in  all  typi- 
cal gneiss,  usually  forming  at  least  one-half  of  the  rock. 
The  orthoclase  may,  however,  be  replaced  to  a  greater 
or  less  extent  by  albitq,  or  even  by  oligoclase.  But  we 
frequently  see  the  term  gneiss  carelessly,  or  ignorantly, 


LITHOLOGY.  99 

applied  to  rocks  which  are  destitute  of  feldspar,  though 
having  the  general  aspect  of  gneiss. 

Augite  rarely  occurs  in  gneiss  ;  and  hence,  when  we 
observe  a  gneiss  containing  a  black  mineral  which  we 
know  is  either  augite  or  hornblende,  it  is  pretty  safe  to 
call  it  the  latter. 

Mica  and  hornblende,  although  the  principal,  are 
not  the  only,  accessory  minerals  in  gneiss  ;  but  the  fol- 
lowing species  are  also  of  common  occurrence  :  garnet, 
cyanite,  tourmaline,  fibrolite,  epidote,  and  chlorite. 
Gneisses,  as  the  table  indicates,  exhibit  a  wide  range 
in  the  proportion  of  silica  which  they  contain,  varying 
from  60  to  85  per  cent. ;  and  there  is  a  concomitant 
variation  in  specific  gravity,  from  about  2.5  in  the  most 
acidic  to  2.8  in  the  most  basic  varieties. 

That  gneiss  is  a  true,  stratified  rock  is  very  clearly 
shown  in  specimen  41 ;  but,  unfortunately,  the  stratifi- 
cation is  not  always  so  evident  as  in  this  case.  The 
mica-scales,  it  will  be  observed,  lie  parallel  with  the 
stratification,  and  assist  very  materially  to  make  it  visi- 
ble ;  and  gneisses  containing  little  or  no  mica,  as  well 
as  some  that  are  rich  in  mica,  frequently  appear  almost 
or  quite  unstratified.  These  obscurely  stratified  varieties 
are  commonly  known  as  granitoid  gneiss,  having  the 
texture  and  general  aspect  of  granite.  The  sediment- 
ary origin  of  gneiss  is  also  clearly  proved  by  its  inter- 
stratification  with  undoubted  sedimentary  rocks,  such 
as  limestone,  iron-ores,  graphite,  quartzite,  etc. 

Syenite.  —  This  is  a  much  abused  term,  but,  as  now 
employed  by  the  best  lithologists,  it  is  the  name  of  a 
rock  having  a  single  essential  constituent,  viz.,  ortho- 
clase.  Syenite  in  its  simplest  variety  contains  nothing 


100  STRUCTURAL    GEOLOGY. 

but  orthoclase ;  but  in  addition  we  usually  have  either 
hornblende,  forming  hornblendic  syenite,  or  mica, 
forming  micaceous  syenite. 

Syenite,  it  will  be  observed,  is  equivalent  to  gneiss 
with  the  quartz  removed  ;  but,  while  gneiss  is  the  most 
abundant  of  all  rocks,  syenite  is  a  comparatively  rare 
rock ;  and  this  is  simply  another  way  of  saying  that 
nearly  all  orthoclase  is  associated  with  quartz.  By 
admixture  of  quartz  we  get  a  perfectly  gradual  passage 
from  syenite  to  gneiss.  The  orthoclase  in  syenite  is 
more  frequently  replaced  by  plagioclase  than  it  is  in 
gneiss.  In  syenite,  too,  hornblende  is  much  more 
abundant  than  mica ;  although  just  the  opposite  is 
true  in  gneiss.  And,  again,  in  gneiss  the  mica  is  prin- 
cipally muscovite  ;  but  in  syenite  it  is  almost  exclusively 
biotite.  Augite  is  a  common  accessory  in  the  more 
basic  syenite ;  but  garnet,  tourmaline,  and  the  other 
accessory  minerals,  occurring  so  frequently  in  gneiss, 
are  almost  unknown  in  syenite.  The  specific  gravity 
of  syenite  varies  from  2.7  to  2.9. 

Diorite.  —  This  is  a  more  important  rock  than  sye- 
nite ;  but  it  is  of  analogous,  though  more  basic,  com- 
position, containing  a  single  essential  constituent,  viz., 
plagioclase.  Any  of  the  triclinic  feldspars  may  occur 
in  this  rock,  but  oligoclase  is  most  common.  Like 
syenite,  diorite  usually  contains  hornblende,  often  in 
large  proportion,  forming  hornblendic  diorite,  which 
sometimes  passes  into  rocks  composed  entirely  of 
hornblende.  It  also,  but  less  frequently,  contains 
mica,  forming  micaceous  diorite.  The  mica  is  usually 
biotite,  rarely  muscovite.  Mica  and  hornblende  also 
often  occur  together  in  diorite,  and  the  same  is  true  of 


LITK&LOC*  101 

syenite  and  gneiss.  Quartz  is  of  common  occurrence 
in  the  more  acidic  varieties  of  diorite,  and  augite  in  the 
more  basic. 

This  is  a  good  example  of  a  basic  rock,  for  all  its 
normal  constituents  are  basic ;  but  the  percentage  of 
silica  varies  from  45  in  those  varieties  richest  in  labra- 
dorite  and  augite  to  60  or  more  in  those  containing 
more  or  less  quartz  and  orthoclase.  There  is  a  corre- 
sponding change  of  color  from  dark  to  light,  and  of 
specific  gravity  from  2.7  to  3.1. 

Diorite  is  not  rich  in  accessory  minerals ;  besides 
those  already  mentioned,  the  most  important  are  chlo- 
rite, epidote,  pyrite,  and  magnetite. 

Few  rocks  are  more  clearly  stratified  than  diorite, 
whether  we  consider  the  hand-specimen,  or  its  relations 
to  other  formations.  It  is  an  abundant  rock  in  New 
England. 

Norite. —  Like  diorite,  this  is  essentially  a  plagio- 
clase  rock ;  but  there  are,  nevertheless,  important  dif- 
ferences. The  plagioclase  in  diorite  is  mainly  the 
more  acidic  species,  like  oligoclase ;  while  in  norite 
the  more  basic  species,  such  as  labradorite  and  an- 
orthite,  predominate.  Hornblende,  which  we  have 
observed  to  be  an  important  and  rather  constant  con- 
stituent of  diorite  and  syenite,  is  much  less  abundant 
in  norite ;  but  its  place  is  taken  by  augite  and  the 
allied  minerals,  hypersthene  and  enstatite.  Black  mica 
is  common  in  norite ;  but  white  mica,  orthoclase,  and 
quartz  rarely  occur. 

Norite  is  the  most  basic  of  all  the  feldspathic  rocks, 
as  gneiss  is  the  most  acidic ;  while  syenite  and  diorite 
stand  as  connecting  links,  forming  a  gradual  passage 


102  STRUCTURAL    GEOLOGY. 

between  the  two  extremes.  Thus,  in  passing  from 
gneiss  to  norite,  we  have  observed  a  gradual  diminu- 
tion of  the  quartz,  a  gradual  change  in  feldspar  from 
orthoclase  to  the  most  basic  plagioclase ;  at  first  a 
gradual  increase  in  hornblende,  and  then  a  gradual 
change  from  hornblende  to  augite ;  and,  finally,  a 
gradual  substitution  of  black  mica  for  white.  The 
amount  of  silica  has  decreased  over  40  per  cent. ;  and 
the  specific  gravity  has  increased  from  2.5  in  the  light- 
est gneiss  to  at  least  3.2  in  the  heaviest  norite.  We 
have  also  passed  from  light  colored  rocks  to  dark ; 
and  from  those  resisting  atmospheric  action  to  those 
easily  decomposed. 

The  most  characteristic  accessory  constituents  of 
norite,  besides  those  already  mentioned,  are  magnetite 
and  chrysolite ;  though  garnet,  serpentine,  and  pyrite 
often  occur.  In  texture,  this  rock  varies  from  compact 
to  very  coarsely  crystalline.  The  specimen  of  labra- 
dorite  (No.  23),  from  the  norite  of  Labrador,  affords 
some  idea  of  the  coarseness  of  the  crystallization  in 
much  of  this  rock.  It  is  not  a  common  rock,  except 
in  certain  regions,  the  best  known  of  which  in  eastern 
North  America  are  the  coast  of  Labrador,  various 
points  in  Canada  north  of  the  St.  Lawrence,  and  the 
eastern  border  of  the  Adirondack  Mountains.  In  hand- 
specimens,  norite  rarely  appears  stratified ;  but  in  the 
solid  ledges  the  stratification  is  often  as  distinct  as 
could  be  desired. 

Many  lithologists  call  the  rocks  here  designated 
norite  gabbro,  and  class  them  all  in  the  eruptive 
division  as  essentially  a  coarse  variety  of  diabase. 


LITHOLCGY.  103 

In  a  similar  manner,  diorite  and  syenite  are  denied  a 
place  in  the  sedimentary  series.  But  the  stratified 
plagioclase  rocks  seem  to  have  as  strong  a  claim  to 
recognition  as  gneiss. 

We  turn  now  to  the  important  and  interesting  divi- 
sion of  the  non-feldspathic  rocks  or  schists, 

Mica  Schist.  —  This  is,  next  to  gneiss,  the  most 
abundant  rock  in  New  England.  Specimen  43  is  a 
typical  example,  and  from  it  we  can  readily  learn  what 
mica  schist  is.  A  glance  suffices  to  show  that  it  is 
chiefly  composed  of  mica,  but  not  entirely;  for,  on 
carefully  examining  the  edges  of  the  specimen,  we  can- 
not fail  to  see  thin  layers  of  hard,  glassy  quartz  inter- 
woven with  the  mica.  The  quartz  layers  are  short  and 
overlapping,  and  we  have  here  a  good  illustration  of 
the  schistose  texture ;  this  is,  in  fact,  a  typical  schist. 

Mica  schist  usually  consists,  as  in  this  instance,  of 
mica  and  quartz ;  but  it  may  be  composed  of  mica 
alone ;  and  sometimes  kaolin  or  clay  takes  the  place 
of  the  quartz,  forming  argillaceous  mica  schist.  The 
mica  in  the  latter  is  usually  in  very  fine  scales  and 
rather  inconspicuous,  and  the  rock  often  passes  into 
ordinary  clay  slate.  Similarly,  when  the  mica  becomes 
deficient  in  the  quartzose  mica  schist,  a  passage  into 
ordinary  quartzite  is  the  result.  A  little  feldspar  is 
sometimes  present  in  the  rock,  which  thus  passes  into 
micaceous  gneiss.  Specimen  43  contains  several  crys- 
tals of  red  garnet,  giving  the  variety  garnetiferous  mica 
schist.  There  is  no  other  rock  that  contains  such  a 
large  variety  of  beautiful  accessory  minerals  as  mica 
schist ;  and  for  the  mineralogist  it  is  one  of  the  most 
attractive  rocks.  Few  rocks  are  more  distinctly  strati- 


104  STRUCTURAL    GEOLOGY. 

fied ;  and  the  stratification  can  usually  be  observed  in 
hand-specimens.  The  mica  in  these  rocks  may  be 
either  muscovite  or  biotite,  or  both ;  but  the  former  is 
most  common.  No  rock  shows  a  greater  variation  in 
the  percentage  of  silica  which  it  contains  than  mica 
schist,  as  we  pass  from  varieties  which  are  nearly  all 
quartz  to  those  which  are  nearly  all  mica. 

Closely  related  to  mica  schist  is  the  rock  now  known 
as  hydromica  schist,  in  which  the  ordinary  anhydrous 
micas  are  replaced  by  hydromica.  It  is  distinguished 
from  mica  schist  by  being  somewhat  softer,  less  harsh 
to  the  touch,  and  less  lustrous.  It  is  to  be  regarded 
usually  as  an  incipient  mica  schist,  which  has  not  yet 
become  anhydrous ;  though  it  may  sometimes  be  just 
the  reverse  ;  viz. :  an  old  mica  schist  which  has  become 
hydrated  through  the  action  of  meteoric  waters.  It 
contains  fewer  accessory  minerals  than  mica  schist. 

Hornblende  Schist. —This  is  a  stratified  aggregate 
of  hornblende  and  quartz.  The  quartz  is  granular  and 
in  thin  layers,  as  in  mica  schist;  but  the  micaceous 
structure  is  wanting,  and  consequently  the  rock  does 
not  cleave  readily  in  the  direction  of  the  bedding. 
The  hornblende  is  mostly  finely  crystalline,  but  some- 
times occurs  in  large,  bladed  crystals.  Garnet  and 
some  other  minerals  are  of  common  occurrence  in  the 
rock ;  but  it  is  not  rich  in  accessories  like  mica  schist. 
The  chief  difficulty  in  recognizing  this  rock  consists  in 
determining  whether  the  white  mineral  is  all  quartz  or 
partly  feldspar.  In  the  latter  case,  of  course,  it  be- 
comes a  hornblendic  gneiss. 

Amphibolite  (Hornblende  Bock).  —  This  is  the  name* 
applied  to  a  rock  having  hornblende  as  its  sole  essen 


LITHOLOGY.  105 

tiai  constituent.  Hornblende  schist  sometimes  passes 
into  amphibolite,  through  the  absence  of  quartz ;  and  so 
does  diorite,  when  the  feldspar  is  deficient  or  wanting. 
Specimen  20,  though  small,  is  a  typical  example  of 
this  rock.  The  physical  and  chemical  characteristics 
are  essentially  the  same  as  for  the  mineral  hornblende. 
The  texture  varies  from  coarsely  to  finely  crystalline. 
The  crystals  are  usually  short  and  thick,  and  lie  in  all 
directions  in  the  rock,  which  is  thus  very  massive,  the 
schistose  texture  being  entirely  wanting,  and  the  strati- 
fication rarely  showing  in  small  masses.  Biotite  is  a 
common  accessory  in  amphibolite,  and  garnet  and 
magnetite  frequently  occur. 

By  the  substitution  of  augite  for  hornblende,  in  the 
description  of  amphibolite,  we  get  the  much  rarer,  but 
otherwise  very  similar,  rock,  pyroxenite. 

Talc  Schist  (Steatite  or  Soapstone).  — Although  not 
abundant,  this  is  a  useful  and  familiar  rock.  The 
composition  is  implied  in  the  name  ;  and  by  comparing 
it  with  the  specimen  of  talc  (No.  58)  we  can  readily 
see  that  they  are  essentially  identical.  Typical  talc 
schist  is  pure  talc ;  but  the  talc  is  often  mixed  with 
more  or  less  quartz  or  feldspar;  and  mica,  chlorite, 
hornblende,  garnet,  and  other  minerals  are  of  common 
occurrence. 

This  rock  embraces  two  distinct  varieties,  the  mas- 
sive and  the  schistose,  or  foliated.  The  former  is  the 
common  soapstone  (specimen  71),  which  is  a  con- 
fused mass  of  crystals  lying  in  all  directions,  and  with 
no  visible  stratification  in  the  small  mass.  In  the  latter, 
as  in  specimen  ,  the  talc  scales  lie  in  parallel  planes, 
giving  the  rock  a  micaceous  structure,  and  causing  it 


106  STRUCTURAL    GEOLOGY. 

to  split  easily  in  the  direction  of  the  stratification.  The 
cleavage  surfaces  are  often  wavy  or  corrugated ;  and 
the  same  is  true  of  all  schistose  rocks.  Talc  schist  is 
easily  distinguished  from  all  other  rocks  by  its  light- 
grayish  or  greenish  color,  combined  with  its  extreme 
softness,  and  its  smooth,  slippery  feel. 

Chlorite  Schist.  — The  one  essential  constituent  of 
this  rock  is  chlorite,  and  the  mineral  specimen  (No.  26) 
answers  equally  well  as  an  example  of  the  rock.  As 
with  talc  schist,  quartz,  feldspar,  and  hydromica  are 
rarely  entirely  absent.  Besides  these,  the  principal 
accessories  are  hornblende,  magnetite,  garnet,  and 
epidote.  This  rock  also  agrees  with  talc  schist  in  pre- 
senting two  principal  varieties,  the  massive  and  the 
schistose.  It  is  easily  distinguished  -from  talc  schist  by 
its  darker  color  and  streak,  which  are  very  character- 
istic ;  while  its  green  color,  softness,  and  unctuous  feel 
separate  it  from  all  other  rocks. 

This  is  the  most  basic  of  all  the  silicate  rocks ;  but, 
in  consequence  of  containing  a  large  proportion  of 
water,  it  is  not  the  heaviest.  It  is,  in  fact,  interesting 
and  important  to  observe  that  all  these  hydrous  silicate 
rocks  —  talc  schist,  chlorite  schist,  green  sand,  and 
serpentine  —  are  distinctly  lighter  in  each  case  than 
anhydrous  rocks  containing  the  same  proportion  of 
silica.  They  are  also  notable,  as  a  class,  for  their  soft- 
ness, smooth  feel,  and  green  color. 

Serpentine.  — As  the  name  implies,  this  rock  is  simply 
the  mineral  serpentine  occurring  in  large  masses,  and 
its  characteristics  are  precisely  the  same.  It  is  fine- 
grained, massive,  compact,  rather  soft,  but  very  tough, 
and  varies  in  color  from  very  dark  green  to  light 


LITHOLOGY.  107 

greenish-yellow.  The  dark  colors  predominate,  and 
specimen  25  is  a  typical  example. 

Serpentine  is  often  intimately  associated  with  lime- 
stone and  dolomite.  The  white  veins  running  irreg- 
ularly through*  the  variety  known  as  Verd  Antique 
Marble,  however,  are  not  calcite,  as  commonly  sup- 
posed, but  magnesite.  They  do  not  effervesce  freely 
with  cold,  dilute  acid,  for  the  entire  rock  is  magnesion, 
and  it  is  probable  have  been  at  one  time  simply  cracks 
along  which  water  holding  carbon  dioxide  has  pene- 
trated, changing  the  magnesia  from  a  silicate  to  a 
carbonate. 

Geologists  were,  at  one  time,  almost  unanimous  in 
the  opinion  that  all  serpentine  is  of  eruptive  origin ; 
but  now  it  .is  conceded  by  the  great  majority  to  be  in 
some  cases  a  sedimentary  rock.  It  is  found  interstrati- 
fied  with  gneiss,  limestone,  all  the  schists,  and  many 
other  stratified  rocks.  When  occupying  the  position 
of  an  eruptive  it  is  never  an  original  rock ;  but  has 
been  formed  by  the  alteration,  in  situ,  of  some  basic 
anhydrous  rock,  most  commonly  olivine  basalt. 

Greensand.  —  This  rock  (specimen  27)  consists 
chiefly  of  the  mineral  glauconite,  mingled  usually  with 
more  or  less  sand,  clay,  or  calcareous  matter.  It  is 
usually  very  friable,  or  in  an  entirely  unconsolidated 
state.  It  is  most  abundant  in  the  newer  geological 
formations,  especially  the  Cretaceous  and  Tertiary ;  and 
is,  perhaps,  the  only  one  of  the  stratified  silicate  rocks 
now  forming  on  an  extensive  scale  in  the  ocean.  Its 
value  as  a  fertilizer,  for  which  purpose  it  is  extensively 
employed,  is  due  to  the  potash  that  it  contains. 

Following  is  a  systematic  summary  of  the  mineral- 


io8 


STRUCTURAL    GEOLOGY. 


ogical  composition  of  the  rocks  of  this  great  division 
of  silicates;  and  this,  combined  with  the  classification 
on  page  70,  presents  in  a  condensed  form  all  the  more 
important  facts  contained  in  the  preceding  descriptions. 
Only  the  more  constant  and  normal  constituents  of  the 
species  are  enumerated  in  each  case  :  — 


Names  of  Species. 

Constituent  Minerals. 

Gneiss    ....    -I 

Orthoclase  and  Quartz. 
Orthoclase,  Quartz,  and  Mica. 
Orthoclase,  Quartz,  and  Hornblende. 

Syenite  .     .     .     .    ) 

Orthoclase. 
Orthoclase  and  Hornblende. 
Orthoclase  and  Mica. 

Diorite   ....-! 

Plagioclase  (chiefly  Oligoclase). 
Plagioclase  and  Hornblende. 
Plagioclase  and  Mica. 

Norite     .     .     .     .    -j 

Plagioclase  (chiefly  Labradorite). 
Plagioclase  and  Augite  (Diallage). 
Plagioclase  and  Mica. 

Mica  Schist     .     .    ) 

Mica. 
Mica  and  Quartz. 
Mica  and  Kaolin. 

Hornblende  Schist  . 

Hornblende  and  Quartz. 

Amphibolite    .     .     . 
Pyroxenite  .... 

Hornblende. 
Pyroxene. 

Talc  Schist     .     .     . 

Talc. 

Chlorite  Schist    .     . 

Chlorite. 

Serpentine      .     .     . 

Serpentine. 

Greensand       .     .     . 

Glauconite. 

LITHOLOGY.  109 

2.    Eruptive  or  Unstratified  Rocks. 

The  rocks  of  this  great  class  are  formed  by  the  cool- 
ing and  solidification  of  materials  that  have  come  up 
from  a  great  depth  in  the  earth's  crust  in  a  melted  and 
highly  heated  condition.  When  the  fissures  in  the 
earth's  crust  reach  down  to  the  great  reservoirs  of 
liquid  rock,  and  the  latter  wells  up  and  overflows  on 
the  surface,  forming  a  volcano,  then  we  may,  as  was 
pointed  out  on  page  33,  divide  the  eruptive  mass  into 
two  parts  :  first,  that  which  has  actually  flowed  out  on 
the  surface,  and  cooled  and  solidified  in  contact  with 
the  air,  forming  a  lava  flow ;  second,  that  which  has 
failed  to  reach  the  surface,  but  cooled  and  solidified 
in  the  fissure,  forming  a  dike. 

Lava  flows  or  volcanic  rocks  and  dikes  or  plutonic 
rocks  are  identical  in  composition  j  but  there  is  a  vast 
difference  in  texture,  due  to  the  widely  different  con- 
ditions under  which  the  rocks  have  solidified.  The 
dike  or  fissure  rocks  solidify  under  enormous  pressure, 
and  this  makes  them  heavy  and  solid  —  free  from  pores. 
They  are  surrounded  on  all  sides  by  warm  rocks  :  this 
causes  them  to  cool  very  slowly,  and  allows  the  various 
minerals  time  to  crystallize.  Other  things  being  equal, 
the  slower  the  cooling  the  coarser  the  crystallization ; 
and  hence,  the  greater  the  depth  below  the  surface  at 
which  the  cooling  takes  place,  the  coarser  the  crystal- 
lization. 

The  volcanic  rock,  on  the  other  hand,  cools  under 
very  slight  pressure  ;  and  the  steam,  which  exists  abun- 
dantly in  nearly  all  igneous  rocks  at  the  time  of  their 
eruption,  is  able  to  expand,  forming  innumerable  small 


HO  STRUCTURAL    GEOLOGY. 

vesicles  or  bubbles  in  the  liquid  lava ;  and  these  remain 
when  it  has  become  solid.  Cooling  in  contact  with  the 
air,  the  lava  cools  quickly,  and  has  but  little  chance 
for  crystallization.  Hence,  to  sum  up  the  matter,  we 
3ay :  plutonic  rocks  are  solid  and  crystalline  ;  and  vol- 
canic rocks  are  usually  porous  or  vesicular,  and 
uncrystalline. 

As  we  descend  into  the  earth's  crust,  it  is  perfectly 
manifest  that  the  volcanic  must  shade  off  insensibly 
into  the  dike  rocks,  and  we  find  it  impossible  to  draw 
any  but  an  arbitrary  plane  of  division  between  them ; 
but  this  is  no  argument  against  this  classification,  for, 
as  already  stated,  all  is  gradation  in  geology,  and  we 
experience  just  the  same  difficulty  in  drawing  a  line 
between  conglomerate  and  sandstone,  or  between  gneiss 
and  mica  schist,  as  between  the  dike  rocks  and  volcanic 
rocks. 

We  will  now  observe  to  what  extent  the  distinc- 
tions between  these  two  great  classes  of  eruptives  can 
be  traced  in  the  rocks  themselves,  beginning  with  the 
dike  rocks.  But  first  it  is  important  to  notice  the 
general  fact,  clearly  expressed  in  the  classification,  that, 
with  perhaps  some  trifling  exceptions  which  need  not 
be  mentioned  here,  all  eruptive  rocks  are  silicates,  and 
nearly  all  are  feldspathic  silicates.  They  are  of  definite 
mineralogical  composition,  and,  like  the  chemically  and 
organically  formed  stratified  rocks,  can  be  classified 
chemically.  But,  although  there  are  eruptives  corres- 
ponding closely  in  composition  to  the  feldspathic  sili- 
cates, which  we  have  just  studied,  we  find  among  them 
little  to  represent  the  non-feldspathic  silicates,  and 
nothing  corresponding  in  composition  to  the  lime- 


LITHOLOGY.  ill 

stones,  dolomites,  gypsum,  flint,  tripolite,  siliceous  tufa, 
iron-ores,  bitumens  or  coals. 

i.  PLUTONIC  (DIKE)  ROCKS.  — These  are  also  known 
as  the  ancient  eruptive  rocks,  and  for  this  reason  :  It 
is  impossible,  of  course,  for  us  to  observe  them  except 
where  they  occur  on  or  near  the  earth's  surface.  But, 
since  they  are  formed  wholly  below  the  surface,  and 
usually  at  great  depths  in  the  earth,  it  is  evident  that 
they  can  appear  on  the  surface  only  as  the  result  of 
enormous  erosion ;  and  erosion  is  a  slow  process,  de- 
manding, in  these  cases,  many  thousands  or  millions 
of  years.  Therefore,  the  more  ancient  dike  rocks  alone 
are  within  our  reach ;  those  of  recent  formation  being 
still  deeply  buried  in  the  earth's  crust.  It  follows,  as 
a  corollary  to  this  explanation,  that  the  coarseness  of 
the  crystallization  of  any  dike  rock  must  be  a  rough 
measure  of  its  age  and  of  the  amount  of  erosion  which 
the  region  has  suffered  since  its  eruption. 

As  regards  composition,  the  dike  rocks  present,  as 
already  stated,  essentially  the  same  combinations  of 
minerals  as  the  feldspathic  silicates  of  the  stratified 
series,  but  occurring  under  different  physical  condi- 
tions and  having  a  widely  different  origin.  The  only 
important  difference  in  texture  between  the  two  classes 
of  rocks  is  that  the  sedimentary  rocks  are  stratified 
and  the  dike  rocks  are  not ;  and  when  we  consider 
that  the  dike  rocks  sometimes  present  a  laminated 
structure  that  resembles  stratification,  while  the  sedi- 
mentary rocks  frequently  appear  unstratified,  it  is  easy 
to  understand  why,  in  the  absence  of  any  marked  dif- 
ference in  composition,  geologists  have  often  found  it 
difficult  to  distinguish  the  two  classes  of  rocks.  We 


112  STRUCTURAL    GEOLOGY. 

also  find  here  the  explanation  and  the  justification  of 
the  fact  that  the  names  of  the  dike  rocks  are  in  most 
cases  the  same  as  those  of  the  sedimentary  rocks  of 
similar  composition. 

Granite.  —  Granite  (from  the  Latin  granum,  a  grain) 
is  a  crystalline-granular  rock,  agreeing  in  composition 
with  gneiss.  The  essential  constituents  are  quartz  and 
orthoclase  ;  and  when  they  alone  are  present  we  have 
the  variety  binary  granite.  Mica,  however  (commonly 
muscovite,  sometimes  biotite,  and  frequently  both)  is 
usually  added  to  these,  forming  micaceous  granite 
(specimen  44)  ;  and  often  hornblende,  forming  horn- 
blendic  granite  (specimen  45).  The  orthoclase  is 
sometimes  replaced  in  part  by  triclinic  species,  espe- 
cially albite  and  oligoclase.  Accessory  minerals  are  not 
so  abundant  in  granite  as  in  gneiss ;  but,  besides  those 
named,  garnet,  tourmaline,  pyrite,  apatite,  and  chlorite 
are  most  common.  Orthoclase  is  always  the  predomi- 
nant ingredient ;  and,  except  when  there  is  much  horn- 
blende present,  usually  determines  the  color  of  the 
granite.  Thus,  specimens  44  and  45  are  gray  because 
they  contain  gray  orthoclase ;  while  all  red  granites 
contain  red  or  pink  orthoclase.  The  quartz  has  usually 
been  the  last  of  the  constituents  to  crystallize  or  solidify ; 
and,  having  been  thus  obliged  to  adapt  itself  to  the  con- 
tours of  the  orthoclase  and  mica,  it  is  rarely  observed 
in  distinct  crystals. 

In  texture,  the  granites  vary  from  perfectly  compact 
varieties,  approaching  petrosilex,  to  those  which  are 
so  coarsely  crystalline  that  single  crystals  of  ortho- 
clase measure  several  inches  in  length.  Of  course 
one  of  the  most  important  things  to  be  observed 


LITHOLOGY.  113 

about  granite,  especially  in  comparing  it  with  gneiss,  is 
the  complete  absence  of  anything  like  stratification ; 
that,  as  before  stated,  being  the  only  important  dis- 
tinction between  the  two  rocks.  Gneiss  is  the  most 
abundant  of  all  stratified  rocks,  and  granite  stands  in 
the  same  relation  to  the  eruptive  series. 

Syenite.  —  This  is  an  instance  where  stratified  and 
eruptive  rocks,  agreeing  in  composition,  have  the  same 
name.  That  rocks  consisting  of  orthoclase,  of  ortho- 
clase  and  hornblende,  or  of  orthoclase  and  mica,  /.<?., 
having  the  composition  of  syenite,  do  occur  in  both 
the  eruptive  and  stratified  series  there  can  be  no  doubt. 
They  should,  however,  have  distinct  names  on  account 
of  their  unlike  origins;  and  would  have  but  for  the 
practical  difficulty  in  determining,  in  many  cases, 
whether  the  rock  is  stratified  or  not.  The  best  that 
we  can  do  now,  when  we  desire  to  be  specific,  and 
have  the  necessary  information,  is  to  say  stratified 
syenite  or  eruptive  syenite,  as  the  case  may  be. 

Diorite.  —  Here,  again,  we  find  identity  of  names, 
as  well  as  of  composition,  between  the  two  great  series. 
Eruptive  diorite  is  an  abundant  and  well  known  rock, 
and  consists  of  the  same  minerals  as  stratified  diorite 
combined  in  the  same  proportions.  Diorite  includes 
a  large  part  of  the  dike  rocks  commonly  known  as 
"  trap  "  and  "  greenstone."  The  principal  accessories 
are  chlorite,  epidote,  pyrite,  magnetite,  apatite,  and 
quartz.  The  texture  varies  from  perfectly  compact  or 
felsitic  to  coarsely  crystalline  ;  averaging,  however,  less 
coarse  than  syenite  and  granite. 

Diabase.  —  By  referring  to  the  classification  it  will  be 
seen  that  diabase  occupies  the  same  position  among 


H4  STRUCTURAL    GEOLOGY. 

the  dike  rocks  as  norite  among  the  stratified  rocks 
Like  norite  it  consists  usually  of  the  more  basic  varie- 
ties of  plagioclase  with  or  without  augite,  diallage,  01 
hypersthene.  Augite,  or  one  of  its  representatives,  is 
usually  present,  and  is  often  the  principal  constituent. 
Specimen  i  shows  a  somewhat  equal  development  of 
the  feldspar  and  augite.  The  name  gabbro  is  some- 
times applied  to  the  coarser  and  more  feldspathic  dia- 
bases, and  especially  to  those  containing  diallage  or 
hypersthene  in  the  place  of  common  augite.  In  the 
opinion  of  some  high  authorities,  however,  it  is  un- 
necessary to  recognize  two  species  here  ;  and  it  makes 
the  classification  more  simple  and  symmetrical  not  to 
do  it.  The  principal  accessories  in  diabase  are  biotite, 
chlorite,  magnetite,  pyrite,  calcite,  and  olivine.  Chlo- 
rite is  often  an  important  constituent,  giving  the  rock 
a  greenish  aspect ;  but  here,  as  well  as  in  diorite,  the 
chlorite  is  due  chiefly  or  entirely  to  the  alteration  of 
the  augite  and  feldspar ;  and  the  chloritic  varieties  of 
diorite  and  diabase  together  make  up  the  old  species 
"  greenstone."  Similarly,  the  more  compact  and  darker 
varieties  of  these  two  rocks,  forming  regular,  wall-like 
dikes,  are  known  as  "  trap."  Specimen  46. 

In  consequence  of  their  more  basic  composition, 
diabase  and  diorite  are  usually  strongly  contrasted  with 
granite  and  syenite  in  color  and  specific  gravity,  being 
darker  and  heavier.  The  basic  rocks,  too,  decay  much 
more  readily  than  the  acidic. 

•*-  2.  VOLCANIC  ROCKS.  —  As  regards  composition,  we 
shall  find  nothing  new  in  the  volcanic  series ;  for  the 
rocks  of  this  group  present  essentially  the  same  com' 
bination  of  minerals  as  the  dike  rocks.  In  composi 


LITHOLOGY.  115 

tion,  the  dike  and  volcanic  rocks  are  identical ;  but  in 
texture,  as  already  explained,  there  is  a  vast  difference. 
The  volcanic  rocks  differ  so  widely  in  texture  from  both 
the  dike  and  stratified  species,  that  there  is  rarely  any 
difficulty  in  distinguishing  them ;  and  hence  they  have 
in  every  instance  distinct  names. 

Volcanic  rocks  are  rarely  found  in  this  part  of  the 
world ;  and  specimens  of  most  of  them  are  difficult 
to  obtain.  For  this  reason  they  can  only  be  noticed 
briefly  here,  since  it  is  the  plan  of  this  Guide  to  give 
especial  attention  only  to  those  portions  of  the  subject 
which  can  be  illustrated  by  material  within  easy  reach 
of  teachers. 

Rhyolite.  —  This  rock  corresponds  in  composition 
with  granite  and  gneiss,  but  is  less  frequently  micaceous. 
The  orthoclase  in  rhyolite,  and  generally  in  volcanic 
rocks,  is  the  clear,  pellucid  variety  —  sanidine.  It  is 
more  difficult  to  separate  from  quartz  than  ordinary 
orthoclase,  the  chief  distinguishing  feature  being  its 
cleavage.  Plagioclase  and  hornblende  are  common, 
but  not  abundant,  constituents.  The  mica,  when  pres- 
ent, is  usually  biotite.  The  texture  of  rhyolite  is  often 
more  or  less  distinctly  porphyritic,  having  a  finely  crys- 
talline or  granular  matrix,  with  interspersed  crystals  of 
sanidine  and  quartz.  The  rock  has  usually  a  rough, 
harsh  feel ;  and  while  the  coarser  varieties  have  the 
aspect  of  granite,  the  finer  approach  petrosilex;  but 
all  are  somewhat  porous,  which  is  seen  in  the  lower 
specific  gravity  of  rhyolite  as  compared  with  granite 
and  gneiss. 

Trachyte.  —  In  texture  and  general  aspect  rhyolite 
and  trachyte  are  nearly  identical.  Trachyte,  however,, 


Il6  STRUCTURAL    GEOLOGY. 

is  darker,  contains  little  or  no  quartz,  and  more  horn- 
blende and  plagioclase.  In  fact,  it  agrees  in  composi- 
tion with  syenite.  This  is  one  of  the  most  important 
of  the  volcanic  rocks. 

Obsidian.  —  Obsidian  is  sharply  distinguished  from 
all  other  rocks  by  its  perfect  vitreous  texture ;  it  is  a 
true  volcanic  glass.  Its  surface  (specimen  47)  is  smooth 
and  glassy,  and  its  fracture  eminently  conchoidal.  To 
the  naked  eye,  and  usually  under  the  microscope,  the 
typical  variety  is  perfectly  homogeneous ;  chemical 
analysis,  however,  shows  that  it  has  the  composition, 
commonly  of  rhyolite,  but  sometimes  of  trachyte. 
Obsidian  is,  in  fact,  simply  rhyolite  or  trachyte  which, 
cooling  quickly,  has  not  had  time  to  crystallize,  but 
has  remained  permanently  in  the  amorphous  or  glassy 
state.  The  composition  is  sometimes  partially  revealed 
where  a  portion  of  the  sanidine  comes  out  in  distinct 
crystals  porphyritically  interspersed  through  the  glass. 
The  homogeneity  of  the  texture  is  sometimes  disturbed: 
by  numerous  minute  concentric  cracks,  forming  what 
is  known  as  perlitic  structure  and  the  variety  perlite ; 
by  numerous  small  spherical  concretions,  forming  the 
spherulitic  structure  and  the  variety  spherulite ;  and 
also  by  the  banding,  which  is  the  result  of  flowing 
while  in  a  plastic  state,  whereby  portions  of  the  glass 
of  slightly  different  colors  are  drawn  out  into  layers  and 
interlaminated.  The  bands  are  rarely  continuous  for 
any  distance,  being  usually  merely  elongated  lenticular 
streaks.  The  glassy  state  is  generally  one  of  inferior 
density,  and  hence  we  find  that  obsidian  is  lighter  than 
the  crystalline  rocks  of  the  same  composition.  Obsid- 
ian is  a  good  illustration  of  a  non-essential  color,  for  its 


LITHOLOGY.  117 

capacity  and  jet-black  color  are  due  entirely  to  impuri- 
ties. In  very  thin  flakes  it  is  transparent  and  white. 
It  also  forms  a  white  powder  when  crushed,  i.e.,  it  has 
a  white  streak. 

Obsidian  is  often  vesicular,  from  the  expansion  of 
the  steam  and  other  gases  which  it  contained  when 
liquid.  The  most  thoroughly  vesicular  varieties  are 
known  as  pumice  (specimen  48).  The  vesicular  tex- 
ture, by  rendering  the  rock  impervious  to  light,  con- 
ceals the  impurities,  and  thus  we  get  a  snow-white 
pumice  from  black  obsidian.  The  vesicles  are  fre- 
quently elongated,  sometimes  in  a  definite  direction, 
though  often  forming  an  irregular  net-work  of  glassy 
fibres.  Pumice  is  often  light  enough  to  float  on  water, 
and  it  is  transported  thousands  of  miles  by  the  oceanic 
currents.  It  is  employed  in  the  arts,  and  good  speci- 
mens can  be  obtained  at  almost  any  drug-store. 

Petrosilex  and  Felsite.  —  Sharply  defined  groups 
are  unknown  in  lithology,  but  all  is  gradation ;  and 
between  rhyolite  and  trachyte,  which  are  always  more 
or  less  distinctly  crystalline,  and  obsidian,  which  is  a 
true  glass  and  perfectly  amorphous,  there  is  no  break. 
It  is  impossible  to  draw  a  sharp  line  and  say,  Here 
the  vitreous  texture  ends  and  the  crystalline  begins ; 
for  the  transition  is  not  abrupt,  but  gradual.  We 
recognize,  really,  in  these  feldspathic  rocks,  an  inter- 
mediate state,  which  is  neither  crystalline  nor  colloid, 
but  both;  and  this  lithologists  have  designated  the 
felsitic  texture.  Felsitic  matter  cannot,  even  with  the 
highest  powers  of  the  microscope,  be  resolved  into 
separate  grains  or  particles ;  and  it  does  not  exhibit, 
except  perhaps  very  indistinctly,  the  phenomenon  of 


Il8  STRUCTURAL    GEOLOGY. 

double  refraction.  In  other  words,  it  is  not  truly  crys« 
talline  or  stony,  and  yet  it  is  just  as  clearly  not  amor- 
phous or  glassy. 

Feldspathic  rocks  exhibiting  the  felsitic  texture  in 
whole  or  in  part  are  known  as  felsites.  Many  high 
authorities  hold  that  true  felsites  are  found  only  among 
the  eruptive  rocks;  while  others  claim  that  they  are 
in  part,  or  wholly,  of  sedimentary  origin.  The  writer 
accepts  the  former  view.  The  felsites  are  in  part  acid 
lavas  which  have  cooled  too  slowly  to  form  a  true  glass, 
like  obsidian,  and  yet  too  quickly  to  become  truly  crys- 
talline, like  rhyolite  and  trachyte.  But  they  are  also 
in  large  part  simply  devitrified  obsidian.  Glass  is  an 
unstable  form  of  mineral  matter;  and  every  species 
of  glass,  including  obsidian,  tends  with  the  lapse  of 
time  to  become  crystalline  or  stony,  the  amorphous 
changing  to  the  felsitic  structure.  Thus,  in  many 
cases  or  usually,  what  we  now  call  felsites  were  origi- 
nally true  glassy  obsidian.  Being  perfectly  intimate 
mixtures  of  the  component  minerals,  the  composition 
of  felsites  can  usually  be  determined  with  certainty 
only  by  means  of  chemical  analysis.  By  this  means 
chiefly,  it  has  been  proved  that  there  are  felsites  agree- 
ing in  composition  with  both  rhyolite  and  trachyte. 
There  is  this  general  difference  in  composition,  how- 
ever, between  these  crystalline  rocks  and  the  felsites ; 
viz. :  mica,  hornblende,  and  augite  are  generally  want- 
ing in  the  latter.  From  this  it  follows  that  the  felsites 
are,  with  unimportant  exceptions,  composed  either  of 
quartz  and  feldspar  or  of  feldspar  alone. 

The  physical  differences  between  the  felsites  of  un- 
like composition  are  not  great ;  but  they  are  sufficient 


LITHOLOGY.  119 

to  warrant  the  division  of  the  felsites  into  two  species : 
a  basic  species,  to  which  the  term  felsites  may  properly 
be  restricted ;  and  an  acidic  species,  for  which  petro- 
silex  is  a  very  appropriate  name.  According  to  this 
arrangement,  felsite  is  composed  chiefly  of  orthoclase, 
and,  as  the  table  shows,  agrees  in  composition  with 
trachyte;  while  petrosilex  consists  mainly  of  ortho- 
clase and  quartz,  agreeing  in  composition  with  rhyo- 
lite.  We  find  here  nothing  new  in  composition ;  but 
petrosilex  and  felsite  are  simply  the  crystalline  rocks 
which  we  have  already  studied,  repeated  under  a  dif- 
ferent texture. 

The  typical  felsite  or  petrosilex  is  composed  entirely 
of  felsitic  matter,  and  is  perfectly  homogeneous,  like 
flint  or  jasper,  which  it  closely  resembles  in  hardness 
and  other  physical  characteristics.  As  a  rule,  however, 
the  rock  is  not  entirely  homogeneous,  but  there  is  a 
manifest  tendency  in  the  component  minerals,  and 
especially  in  the  feldspar,  to  separate  out,  usually  in 
the  form  of  crystals.  In  the  banded  variety  (specimen 
42)  the  rock  is  built  up  of  thin  layers,  which  are  often 
alternately  quartzose  and  feldspathic.  There  is  not  a 
perfect  separation  of  the  minerals  ;  but  that  the  quartz 
is  chiefly  in  the  dark  layers,  and  the  feldspar  in  the 
light,  is  shown  by  the  way  in  which  the  layers  are 
affected  by  the  weather. 

One  of  the  most  common  varieties  is  where  a  por- 
tion, frequently  a  large  portion,  of  the  feldspar  comes 
out  in  the  form  of  distinct,  separate  crystals,  produc- 
ing a  porphyritic  texture.  Specimens  5,  6,  and  7  are 
examples  of  porphyritic  felsite ;  and  after  examining 
these  we  can  no  longer  doubt  that  feldspar  is  an  im- 


120  STRUCTURAL    GEOLOGY. 

portant  constituent  of  the  rock.  Petrosilex  and  felsite 
are  more  generally  porphyritic  than  any  other  rocks ; 
and  they  are  commonly  called  porphyry.  It  is  better, 
however,  since  almost  any  rock  may  be  porphyritic, 
and  since  this  texture  cannot  be  correlated  with  any 
particular  composition,  not  to  use  porphyry  as  a  rock- 
name,  but  simply  as  the  name  of  a  very  important 
rock-texture.  The  banded  and  porphyritic  textures 
are  about  equally  characteristic  of  petrosilex  and  felsite. 
In  petrosilex,  quartz,  as  well  as  feldspar,  is  sometimes 
porphyritically  developed,  forming  the  variety  known 
as  quartz-porphyry.  There  is  no  limit  to  the  propor- 
tion of  the  quartz  and  feldspar  which  may  crystallize 
out  in  this  way,  and  thus  we  find  a  perfectly  gradual 
passage  from  normal  petrosilex  or  felsite  to  thoroughly 
crystalline  granite  and  syenite. 

Andesite.  —  This  rock  has  nearly  the  texture  of  rhy- 
olite  and  trachyte,  but  is  darker  and  heavier,  and 
corresponds  in  composition  to  diorite,  consisting  of 
plagioclase  and  hornblende,  with  usually  more  or  less 
sanidine,  quartz,  augite,  biotite,  and  magnetite. 

Basalt. — The  rock  bearing  this  familiar  name  repre- 
sents diabase  among  the  dike  rocks.  It  is  the  most 
basic  of  the  volcanic  rocks,  and  consists  of  the  more 
basic  varieties  of  plagioclase,  especially  labradorite, 
with  augite,  magnetite,  and  titanic  iron.  Olivine  is  a 
very  common  and  characteristic  constituent,  and  the 
plagioclase  is  often  replaced  in  part  by  leucite  and 
nephelite.  The  basalts  are  usually  black,  and  of  high 
specific  gravity ;  and  vary  in  texture  from  compact  to 
coarsely  crystalline.  The  contraction  due  to  cooling 
frequently  results  in  the  development  of  a  columnar 


LIT  HO  LOGY.  121 

structure  of  remarkable  regularity,  the  columns  being 
normally  hexagonal  and  standing  perpendicularly  to 
the  cooling  surfaces  of  the  mass.  This  structure  occurs 
in  other  eruptive  rocks,  but  is  most  characteristic  of 
basalt. 

Tachylite.  —  Tachylite  is  a  highly  basic  volcanic 
glass,  standing  in  the  same  relation  to  basalt  and  ande- 
site  that  obsidian  does  to  trachyte  and  rhyolite.  It  is 
much  heavier  than  obsidian,  and  is  perfectly  black  and 
opaque,  except  in  the  finest  fibres.  It  is  a  compara- 
tively rare  rock,  for  the  reason  that  basalt  and  andesite 
crystallize  more  readily  than  the  acidic  rocks  on  pass- 
ing from  the  liquid  to  the  solid  state.  On  the  surface 
of  the  basic  lava,  however,  where  it  is  in  contact  with 
the  air,  and  congeals  almost  instantly,  a  film  of  glass 
is  formed  ;  but  this  may  not  be  more  than  a  small  frac- 
tion of  an  inch  in  thickness.  Like  obsidian,  tachylite 
is  often  vesicular ;  but  the  vesicular  basic  rocks,  as  well 
as  the  solid,  are  usually  stony.  They  occur  in  vast 
abundance  in  many  volcanic  regions,  and  may  be  con- 
sidered the  typical  lava  (specimen  49). 

In  the  more  ancient  lavas,  the  vesicles  are  frequently 
filled  by  various  minerals  —  chlorite,  epidote,  quartz, 
calcite,  etc.  —  deposited  by  infiltrating  waters,  and  de- 
rived in  most  cases  from  the  decomposition  of  the 
original  constituents  of  the  rock.  Thus  the  vesicular 
is  changed  to  the  amygdaloidal  texture,  and  the  lava 
becomes  an  amygdaloid  (specimen  50).  The  amyg- 
daloidal texture  is  common  in  the  basic  lavas  and  rare 
in  pumice,  simply  because  the  former  are  more  readily 
decomposed  and  contain  a  greater  variety  of  bases  from 
which  secondary  minerals  can  be  formed. 


122  STRUCTURAL    GEOLOGY, 

Porphyrite  and  Melaphyr.  —  These  two  rocks  hold 
essentially  the  same  relation  as  regards  origin  and 
structure  to  the  basic  lavas  that  petrosilex  and  felsite 
do  to  the  acidic  lavas.  Porphyrite  agrees  in  compo- 
sition with  andesite,  and  melaphyr  with  basalt.  They 
are  usually  dark-colored  rocks  having  a  compact  or 
felsitic  texture.  Porphyrite  is,  as  the  name  implies, 
-rery  commonly  porphyritic;  while  melaphyr  is  often 
vesicular  or  brecciated,  exhibiting  all  the  structural 
features  of  tachylite  and  basalt,  and  being  in  its  older 
forms  very  generally  amygdaloidal. 

Volcanic  Tuff  and  Agglomerate.  —  Besides  the  crys- 
talline, glassy,  and  felsitic  lavas,  already  described,  and 
due  chiefly  to  the  rate  of  cooling  of  the  liquid  rock, 
we  may  recognize  a  fourth  class  to  include  the  very 
abundant  lavas  which,  (luring  explosive  eruptions,  are 
ejected  in  the  solid  state,  being  violently  blown  out  of 
the  crater  in  the  form  of  dust  and  fragments.  Falling 
on  the  slopes  of  the  volcano  or  over  the  surrounding 
country,  as  in  the  case  of  the  buried  city  of  Pompeii, 
the  fragmental  lavas  remain  largely  unstratified.  But 
when,  as  frequently  happens,  they  fall  into  the  sea, 
they  are  assorted  by  the  waves  and  currents  and 
arranged  in  layers  after  the  manner  of  ordinary  sedi- 
ments, with  which  they  are  often  more  or  less  mixed. 
Before  they  become  consolidated  the  finer  fragmental 
lava,  of  whatever  composition,  is  called  volcanic  dust, 
and  the  coarser  lapilli  or  volcanic  sand;  while  the 
consolidated  materials  are  known  as  tuff  and  agglom- 
erate respectively. 


SUPPLEMENT  TO   LITHOLOGY. 


VEIN    ROCKS. 

ALL  rocks  are  not  embraced  in  the  sedimentary  and 
eruptive  divisions,  but  there  is  a  third  grand  division, 
which,  although  rarely  mentioned  or  recognized  in  the 
more  comprehensive  works  on  geology,  it  is  deemed  best 
not  to  leave  entirely  unnoticed  here.  These  are  the  vein 
rocks.  They  present  an  immense  number  of  varieties,  and 
yet,  taken  altogether,  form  but  a  small  fraction  of  the 
earth's  crust.  They  are,  however,  the  great  repositories 
of  the  precious  and  other  metals,  and  hence  are  objects  of 
far  greater  interest  to  the  miner  and  practical  man  than  the 
eruptive  rocks,  or,  in  some  parts  of  the  world,  even  than 
the  sedimentary  rocks. 

The  vein  rocks,  like  the  eruptive  rocks,  occupy  fissures 
in  the  earth's  crust  intersecting  the  stratified  formations ; 
but  the  fissures  filled  with  vein  rocks  are  called  veins,  and 
not  dikes.  We  will  first  notice  the  mode  of  formation  of 
a  typical  vein,  and  then  examine  its  contents.  Geologists 
are  agreed  that  water  penetrates  to  a  very  great  depth  in 
the  earth's  crust.  All  minerals  are  more  or  less  soluble 
in  water;  and  we  may  consider  the  water  circulating 
through  the  rocks,  especially  at  considerable  depths,  as, 
in  most  cases,  a  saturated  solution  of  the  various  minerals 
of  which-  they  are  composed.  Very  slight  changes  in  the 
conditions  will  cause  saturated  solutions  to  deposit  part  of 
their  mineral  load.  The  water  at  great  depths  has  a  high 
temperature,  and  is  subjected  to  an  enormous  pressure; 


124  STRUCTURAL   GEOLOGY. 

and  both  of  these  circumstances  favor  solution.  Suppose, 
now,  that  these  hot  subterranean  waters  enter  a  fissure  in 
the  crust  and  flow  upwards,  perhaps  issuing  on  the  surface 
as  a  warm  mineral  spring ;  as  they  approach  the  surface, 
the  temperature  and  pressure,  and  consequently  their  sol- 
vent power,  are  diminished ;  and  a  portion  of  the  dis- 
solved- minerals  must  be  deposited  on  the  walls  of  the 
fissure,  which  thus  becomes  narrower,  and  in  the  course 
of  time  is  gradually  filled  up.  The  vein  is  then  complete  ; 
and  the  mineral  waters  are  forced  to  seek  a  new  outlet. 

Veins  have  the  same  general  forms  as  dikes,  since  the 
fissures  are  formed  in  the  same  way  for  both ;  but  the 
vein  is  of  slow  growth,  and  may  require  ages  for  its 
completion,  while  the  dike  is  formed  in  an  hour  or  a  day. 
It  is  now  generally  believed  that  water  is  an  important 
agent  in  the  formation  of  eruptive  rocks ;  since  they  all 
contain  water  at  the  time  of  their  eruption ;  and  since  it 
has  been  demonstrated  that,  while  ordinary  rocks  require 
a  temperature  of  2000°  to  3000°  for  their  fusion  in  the  ab- 
sence of  water,  they  are  liquified  at  temperatures  below 
1000°  in  the  presence  of  water,  In  other  words,  common 
rocks  are  very  infusible  and  insoluble  bodies,  and  heat  and 
water  acting  independently  have  little  effect  upon  them ; 
but  when  fire  and  water  are  combined  in  what  is  now 
known  as  aqueo-igneous  fusion,  they  prove  very  efficient 
agents  of  liquefaction. 

If  we  adopt  these  views,  then  it  can  be  shown  that,  in 
origin,  veins  and  dikes  differ  in  degree  only,  and  are  not 
fundamentally  unlike  ;  and  the  formation  and  relations  of 
the  three  great  classes  of  rocks  may  be  summarized  as 
follows :  — 

The  ocean  and  atmosphere,  operating  on  the  earth's  sur- 
face, have  worked  over  and  stratified  the  crust/ until  the 
sedimentary  rocks  have  now  an  average  thickness  variously 
estimated  at  from  ten  to  thirty  or  forty  miles.  This  entire 
thickness  of  stratified  rocks,  and  a  considerable  depth  of 


VEIN  ROCKS.  125 

the  underlying  unstratified  crust,  must  be  saturated  with 
water;  and  all  but  the  more  superficial  portions  of  this 
water-soaked  crust  must  be  very  hot,  the  temperature 
increasing  steadily  downwards  from  the  surface.  Both 
eruptive  and  vein  rocks  originate  in  this  highly  heated, 
hydrated  crust.  Eruptive  rocks  are  formed  when  the  heat, 
aided  by  more  or  less  water,  softens  the  rocks,  either 
stratified  or  unstratified,  by  aqueo-igneous  fusion,  and  the 
plastic  materials  are  forced  up  through  fissures  to  or  to- 
ward the  surface.  Vein  rocks  are  formed  when  the  water, 
aided  by  more  or  less  heat,  dissolves  the  rocks,  either 
stratified  or  unstratified,  by  what  may  be  called  igneo- 
aqueous  solution,  and  subsequently  deposits  the  mineral 
matter  in,  i.e.,  on  the  walls  of,  fissures  leading  up  to  or 
toward  the  surface.  In  the  case  of  the  dike  rocks,  heat  is 
the  chief  agent,  and  water  merely  an  auxiliary  ;  while  with 
the  vein  rocks  it  is  just  the  reverse.  But  between  the  two 
it  is  probably  impossible  to  draw  any  sharp  line. 

The  water  circulating  through  the  crust,  and  saturated 
with  its  various  mineral  constituents,  has  been  called  the 
"juice"  of  the  crust;  and  veins  are  formed  by  the  con- 
centration of  this  earth-juice  in  fissures.  One  of  the  most 
important  characteristics  of  the  vein  rocks,  as  a  class,  is 
the  immense  variety  which  they  present ;  for  nearly  every 
known  mineral  is  embraced  among  their  constituents  ;  and 
these  are  combined  in  all  possible  ways  and  proportions, 
so  that  the  number  of  combinations  is  almost  endless. 
The  solvent  power  of  the  subterranean  waters  varies  for 
different  minerals;  and  appears  often  to  be  greatest  for 
the  rarer  species.  In  other  words,  there  is  a  sort  of  selec- 
tive action,  whereby  many  minerals  which  exist  in  strati- 
fied and  eruptive  rocks,  so  thinly  diffused  as  to  entirely 
escape  the  most  refined  observation,  are  concentrated  in 
veins  in  masses  of  sensible  size ;  and  our  lists  of  known 
minerals  and  chemical  elements  are  undoubtedly  much 
longer  than  they  would  be  if  these  wonderful  storehouses 


126  STRUCTURAL  GEOLOGY. 

of  fine  minerals  which  we  call  veins  had  never  been 
explored.  As  a  rule,  the  minerals  in  veins  form  larger 
and  more  perfect  crystals  than  we  find  in  either  of  the 
other  great  classes  of  rocks.  This  is  simply  because  the 
conditions  are  more  favorable  for  crystallization  in  veins 
than  in  dikes  or  sedimentary  strata.  In  both  dike  and 
stratified  rocks,  the  growing  crystals  are  surrounded  on  all 
sides  by  solid  or  semi-solid  matter ;  and,  being  thus  ham- 
pered, it  is  simply  impossible  that  they  should  become 
either  large  or  perfect.  In  the  vein,  on  the  other  hand, 
there  are  usually  no  such  obstacles  to  be  overcome ;  but 
the  crystals,  starting  from  the  walls  of  the  fissure,  grow 
toward  its  centre,  their  growing  ends  projecting  into  a 
free  space,  where  they  have  freedom  to  develop  their  nor- 
mal forms  and  to  attain  a  size  limited  only,  in  many  cases, 
by  the  breadth  of  the  fissure.  With,  possibly,  some  rare 
exceptions,  all  the  large  and  perfect  crystals  of  quartz, 
feldspar,  mica,  beryl,  apatite,  fluorite,  and  of  minerals 
generally,  which  we  see  in  mineralogical  cabinets,  have 
originated  in  veins  Those  fissures  which  become  the 
seats  of  mineral  veins  are  really  Nature's  laboratories  for 
the  production  of  rare  and  beautiful  mineral  specimens ; 
and  hence  the  vein  rocks  are  the  chief  resort  of  the  miner- 
alogist, to  whom  they  are  of  far  greater  interest  than  all 
the  eruptive  and  stratified  rocks  combined. 

The  leading  characteristics,  then,  of  the  vein  rocks  may 
be  summarized  as  follows:  (i)  They  contain  nearly  all 
known  minerals,  including  many  rare  species  and  elements 
which  are  unknown  outside  of  this  class  of  rocks.  (2) 
These  mineral  constituents,  occurring  singly  and  com- 
bined, give  rise  to  a  number  of  varieties  of  rocks  so  vast 
as  to  baffle  detailed  description.  (3)  They  exceed  all 
other  rocks  in  the  coarseness  of  their  crystallization,  and 
in  the  perfection  and  beauty  of  the  single  crystals  which 
they  afford. 


PETROLOGY.  127 

PETROLOGY. 

In  lithology  we  investigate  the  nature  of  the  mate- 
rials composing  the  earth's  crust  —  the  various  min- 
erals and  aggregates  of  minerals,  or  rocks;  while  in 
petrology  we  consider  the  forms  and  modes  of  arrange- 
ment of  the  rock-masses,  —  in  other  words,  the  archi- 
tecture of  the  earth. 

Petrology  is  the  complement  of  lithology,  and  in 
many  respects  it  is  the  most  fascinating  division  of 
geology,  since  in  no  other  direction  in  this  science  are 
we  brought  constantly  into  such  intimate  relations  with 
the  beautiful  and  sublime  in  nature.  The  structures  of 
rocks  are  the  basis  of  nearly  all  natural  scenery ;  for 
what  we  call  scenery  is  usually  merely  the  external 
expression,  as  developed  by  the  powerful  but  delicate 
sculpture  of  the  agents  of  erosion  —  rain  and  frost, 
rivers  and  glaciers,  etc.  —  of  the  geological  structure 
of  the  country.  And  to  the  practised  eye  of  the  geolo- 
gist, a  fine  landscape  is  not  simply  a  pleasantly  or 
grandly  diversified  surface,  but  it  has  depth;  for  he 
reads  in-  the  superficial  lineaments  the  structure  of  the 
rocks  out  of  which  they  are  carved. 

But,  while  the  magnitude  of  the  phenomena  adds 
greatly  to  the  charm  of  the  study,  it  also  increases  the 
difficulties  and  taxes  the  ingenuity  of  the  teacher  whose 
work  must  be  done  indoors.  According  to  our  ideal 
method,  natural  science  ought  to  be  taught  with  nat- 
ural specimens ;  and  yet  here  our  main  reliance  must 
be  upon  pictures  and  diagrams. 

Nature,  however,  has  not  been  wholly  unmindful  oi 
our  needs;  for  she  has  worked  often  upon  a  verf 


128  STRUCTURAL    GEOLOGY. 

small  as  well  as  a  very  large  scale ;  many  of  the  grand- 
est  phenomena  being  repeated  in  miniature.  Thus 
we  observe  rock-folds  or  arches  miles  in  breadth  and 
forming  mountain  masses,  and  of  all  sizes  from  that 
down  to  the  minutest  wrinkle.  So  with  veins,  faults, 
etc.  And  the  wonderful  thing  is  that  these  small  ex- 
amples, which  may  be  brought  into  the  class-room, 
are  usually,  except  in  size,  exactly  like  the  large.  Now 
the  aim  of  every  teacher  in  this  department  should  be 
to  secure  a  collection  of  these  natural  models.  It  is 
not  an  easy  thing  to  do,  except  one  has  plenty  of 
time ;  for  they  can  rarely  be  purchased  of  dealers,  but 
must  usually  come  as  the  choicest  fruit  of  repeated 
excursions  to  the  natural  ledges  and  quarries,  the  sea- 
shore and  the  mountains.  But  for  the  difficulty  of 
getting  the  specimens  there  is  some  compensation, 
since  it  may  be  truly  said  that  for  the  collector  speci- 
mens obtained  in  this  way  have  an  interest,  a  value, 
and  a  power  of  instruction  beyond  what  they  would 
otherwise  possess. 

Classification  of  Structures. 

The  structures  of  rock  divide,  at  the  outset,  into  two 
classes  :  —  ( i )  the  original  structures,  or  those  pro- 
duced at  the  same  time  and  by  the  same  forces  as  the 
rocks  themselves,  and  which  are,  therefore,  peculiar 
to  the  class  of  rocks  in  which  they  occur  (e.g.  stratifi- 
cation, ripple-marks,  fossils,  etc.)  ;  and  (2)  the  sub- 
sequent structures,  or  those  developed  in  rocks  subse- 
quently to  their  formation,  and  by  forces  that  act  more 
or  less  uniformly  upon  all  classes  of  rocks,  and  which 


PETROLOGY. 


129 


are,  therefore,  in  a  large  degree,  common  to  all  kinds 
of  rocks  (e.g.  folds,  faults,  joints,  etc.). 

The  original  structures  are  conveniently  and  natu- 
rally classified  in  accordance  with  the  three  great  classes 
of  rocks:  (i)  stratified  rocks,  (2)  eruptive  rocks,  and 
(3)  vein  rocks ;  while  the  subsequent  structures,  not 
being  peculiar  to  particular  classes  of  rocks,  are  prop- 
erly divided  into  those  produced  by  (i)  the  subter- 
ranean or  igneous  agencies,  and  (2)  the  superficial  or 
aqueous  agencies. 


mflm'fflih  *,fir  $fifr\ I'nfoi  fr-Mpt  T  f gff^qf'V^'-''** 


Fig.  i.  —  Section  through  sediment  deposited  by  rain  in  a  roadside  pool; 
a.  surface  of  roadway;  b,  layer  of  small  pebbles  and  coarse  sand; 
c.  fine  sand  passing  into  d',  d.  the  finest  sand  and  mud. 


Original  Structures  of  Stratified  Hocks* 

STRATIFICATION.  —  All  rocks  formed  by  strewing 
materials  in  water,  and  their  deposition  in  succes- 
sive, parallel,  horizontal  layers,  are  stratified;  and  this 
structure  is  their  stratification.  It  is  the  most  import- 
ant of  all  rock  structures ;  and  there  is  no  kind  of 
structure  the  origin  of  which  is  more  fully  or  certainly 


1 30  STRUCTURAL    GEOLOGY. 

known.  The  deposition  of  sediment  in  carefully  assorted 
horizontal  layers  is  readily  brought  within  the  compre- 
hension of  children  by  simple  experiments  with  sand 
and  clay  in  water ;  and  still  better  by  the  examination 
of  the  deposits  formed  in  roadside  pools  during  heavy 
rains  (Fig.  i),  and  by  digging  into  beaches  and  sand- 
bars, which  every  child  will  recognize  as  formed  of 
materials  arranged  by  water.  Great  stress  should  be 
laid  upon  the  fact  that  a  lake  like  Erie  or  Champlain 
is  simply  a  large  pool  with  several  more  or  less  turbid 
streams  flowing  into  it,  while  the  single  stream  flowing 
out  is  clear,  the  sediment  having  evidently  been  de- 
posited in  the  lake;  and  that  every  lake  is,  like  the 
roadside  pool,  being  gradually  filled  up  with  sedimen- 
tary or  stratified  rocks.  But  the  ocean  is  a  still  larger 
pool,  receiving  mud  and  sand  from  many  streams  ;  and 
since  we  know  that  nothing  escapes  from  the  ocean  but 
invisible  vapor,  it  is  plain  that  the  mud  and  sand  and 
all  other  kinds  of  sediment  carried  into  the  ocean  must 
be  deposited  on  its  floor,  and  chiefly,  as  we  have  seen, 
on  that  part  nearest  the  land.  The  consolidation  of 
beaches,  bars,  and  mud-flats  is  all  that  is  necessary  to 
convert  them  into  stratified  formations  of  conglomerate 
sandstone  and  slate. 

Let  us  notice  now,  more  particularly,  the  causes  of 
visible  stratification.  As  we  can  easily  prove  by  an 
experiment  with  clay  in  a  bottle  of  water,  if  the  same 
kind  of  material  is  deposited  continuously  there  will 
be  no  visible  stratification  in  the  deposit.  It  will  be 
as  truly  stratified  as  any  formation,  but  not  visibly  so  ; 
because  there  is  nothing  in  the  nature  of  the  material 
or  the  way  in  which  it  is  laid  down  to  bring  out  distinct 


PETROLOGY.  13! 

lines  of  stratification.  Continuous  and  uniform  deposi- 
tion obtains  very  frequently  in  nature,  but  rarely  con- 
tinues long  enough  to  permit  the  formation  of  thick 
beds  or  strata.  Hence,  while  the  stratification  is  almost 
always  visible  on  the  large  surfaces  of  sandstone,  slate, 
etc.,  exposed  in  quarries  and  railway  cuttings,  and 
may  usually  be  seen  in  the  quarried  blocks,  it  is  often 
not  apparent  in  hand  specimens,  which  may  represent 
a  single  homogeneous  layer.  There  is  one  important 
exception,  and  that  is  where  the  particles,  although  of 
the  same  kind,  are  flat  or  elongated.  Pebbles  of  these 
forms  are  common  on  many  beaches ;  and  since  they 
are  necessarily  arranged  horizontally  by  the  action  of 
the  water,  they  willj  by  their  parallelism,  make  the 
stratification  of  the  pudding-stone  visible.  The  same 
result  is  accomplished  still  more  distinctly  by  the  mica 
scales,  etc.,  in  sandstone  and  slates,  the  leaves  and 
flattened  stems  of  vegetation  in  bituminous  coal,  and 
the  flat  shells  in  limestone. 

In  all  other  cases,  visible  stratification  implies  some 
change  in  the  conditions;  either  the  deposition  was 
interrupted,  or  different  kinds  of  material  were  de- 
posited at  different  times.  The  first  cause  produces 
planes  of  easy  splitting,  or  fissility,  especially  in  fine- 
grained rocks,  like  shale.  This  shaly  structure  or 
lamination-cleavage  may  be  due,  in  some  cases,  to 
pressure,  but  it  is  commonly  understood  to  mean  that 
each  thin  layer  of  clay  became  partially  consolidated 
before  the  next  one  was  deposited  upon  it,  so  that  the 
two  could  not  perfectly  cohere.  Parallel  planes  of 
easy  splitting  are,  however,  by  themselves,  of  little 
value  as  indications  of  stratification,  since  the  lamina- 


*32  STRUCTURAL    GEOLOGY. 

tion-cleavage  is  not  easily  distinguished  from  slaty- 
cleavage  (roofing  slate)  and  parallel  jointing,  structures 
developed  subsequently  to  the  deposition  of  sediments 
and  quite  independent  of  the  stratification.  The  sec- 
ond cause,  or  variations  in  the  kind  of  sediment,  gives 
alternating  layers  differing  in  color,  texture,  or  compo- 
sition, as  is  seen  frequently  in  sandstone,  slate,  gneiss, 
etc. ;  and  of  all  the  indications  of  stratification  these 
are  the  most  important  and  reliable. 


Fig.  2. —  Section  showing  strata  and  laminae:  a.  conglomerate;  b.  sandstone; 
c.  shale ;  d.  limestone. 

A  layer  composed  throughout  of  essentially  the  same 
kind  of  rock,  as  conglomerate  or  sandstone,  and  show- 
ing no  marked  planes  of  division,  is  usually  regarded 
as  one  bed  or  stratum,  although  it  may  vary  consider- 
ably in  texture  or  color;  while  the  thinner  portions 
composing  the  stratum  and  differing  slightly  in  color, 
texture,  and  composition,  and  the  thin  sheets  into 
which  shaly  rocks  split,  are  the  lamina  or  leaves*  In 


PETROLOGY. 

Fig.  2  the  strata  are  designated  by  letters,  and  the  fine 
lines  and  rows  of  dots  show  the  constituent  laminae,  while 
the  whole  section  may  be  regarded  as  a  small  part  of 
a  great  geological  formation.  The  geological  record 
is  written  chiefly  in  the  sedimentary  rocks ;  and  the 
formations,  strata,  and  laminae  may  be  regarded  as  the 
volumes,  chapters,  and  pages  in  the  history  of  the 
earth.  Now  every  feature  of  a  rock,  lithological  or 
petrological,  finds  its  highest  interest  in  the  light  which 
it  throws  upon  the  history  of  the  rock,  i.e.,  upon  the 
conditions  of  its  formation.  Observe  what  the  section 
in  Fig.  2  teaches  concerning  the  geological  history  of 
that  locality ;  premising  that  any  chapter  of  geologi- 
cal history  written  in  the  stratified  rocks  should  be 
read  from  the  bottom  upwards,  since  the  lowest  strata 
must  have  been  formed  first  and  the  highest  last.  The 
lowest  stratum  exposed  is  conglomerate,  indicating  a 
shingle  beach  swept  by  strong  currents  which  carried 
away  the  finer  material.  Upwards,  the  conglomerate 
becomes  finer  and  shades  off  into  sandstone,  and  finally 
into  shale,  showing  that  the  water  has  become  gradually 
deeper  and  more  tranquil,  the  shore  having,  in  conse- 
quence of  the  subsidence,  advanced  toward  the  land. 
The  next  two  strata  show  that  this  movement  is  prob- 
ably reversed ;  at  any  rate,  the  currents  become  stronger 
again,  and  the  shale  passes  gradually  into  sandstone 
and  conglomerate.  The  beach  condition  prevails  now 
for  a  long  time,  and  thick  beds  of  sand  and  gravel  are 
formed.  The  sea  then  deepens  again,  and  we  observe 
a  third  passage  from  coarse  to  fine  sediment.  This 
locality  is  now  remote  from  the  shore,  the  gentle  cur- 
rents bringing  only  the  finest  mud,  which  slowly  builds 


*34  STRUCTURAL    GEOLOGY. 

up  the  thick  bed  of  shale,  in  the  upper  part  of  which 
shells  are  abundant,  indicating  that  the  deposition  of 
mechanical  sediment  has  almost  ceased,  and  that  the 
shale  is  changing  to  limestone.  The  purity  of  the 
limestone,  and  the  crinoids  and  other  marine  organ- 
isms which  it  contains,  prove  that  this  has  now  become 
the  deep,  clear  sea ;  and  this  condition  is  maintained 
for  a  long  period,  for  the  limestone  is  very  thick,  and 
this  rock  is  formed  with  extreme  slowness. 

The  most  important  point  to  be  gained  here  is  that 
every  line  of  stratification  and  every  change  in  the 
character  of  the  sediments  is  due  to  some  change  of 
corresponding  magnitude  in  the  conditions  under  which 
the  rock  was  formed.  The  slight  and  local  changes 
in  the  conditions  occur  frequently  and  mark  off  the 
individual  laminae  and  strata,  while  the  more  important 
and  wide-spread  changes  determine  the  boundaries  of 
the  groups  of  strata  and  the  formations. 

Strata  are  subject  to  constant  lateral  changes  in  tex- 
ture and  composition,  i.e.,  a  bed  or  formation  rarely 
holds  the  same  lithological  characteristics  over  an  ex- 
tended area.  There  are  some  striking  exceptions, 
especially  among  the  finer-grained  rocks,  like  slate, 
limestone,  and  coal,  which  have  been  deposited  under 
uniform  conditions  over  wide  areas.  It  is  the  general 
rule,  however,  particularly  with  the  coarse-grained 
rocks,  which  have  been  deposited  in  shallow"  water 
near  the  land,  that  the  same  continuous  stratum  under- 
goes great  changes  in  thickness  and  lithological  char- 
acter when  followed  horizontally.  A  stratum  of  con- 
glomerate becomes  finer  grained  and  gradually  changes 
into  sandstone,  which  shades  off  imperceptibly  into 


PETROLOGY. 


'35 


slate,  and  slate  into  limestone,  etc.  Where  the  stratum 
is  conglomerate,  its  thickness  will  usually  be  much 
greater  and  more  variable  than  where  it  is  composed 
of  the  finer  sediments.  The  rapidity  of  these  changes 
in  certain  cases  is  well  shown  by  the  parallel  sections 
in  Fig.  3.  These  repre- 
sent precisely  the  same 
beds,  as  the  connecting 
lines  indicate,  at  points 
only  twenty  feet  apart. 

When  we  glance  at 
the  conditions  under 
which  stratified  rocks 
are  now  being  formed, 
it  is  plain  that  all  strata 
must  terminate  at  the 
margin  of  the  sea  in 
which  they  were  de- 
posited, and  in  the  mar: 
ginal  portions  of  that 
sea,  especially,  must 
exhibit  frequent  and 
rapid  changes  in  com- 
position, etc.  The  sedi- 
ments forming  the  sur- 
face of  the  sea-bottom 


Fig.  3.  —  Parallel  sections  showing  rapid 
lateral  changes  in  strata:  c.  clay; 
s.  sand;  ss,  sandstone;  /.  lignite; 
/.  fireclay. 


at  the  present  time  may 
be  regarded  as  belonging  to  one  continuous  stratum ; 
and  it  is  instructive  to  examine  a  chart  of  any  part  of 
our  coast,  such  as  Massachusetts  Bay,  on  which  the 
nature  of  the  bottom  is  indicated,  for  each  sounding, 
and  observe  the  distribution  of  the  different  kinds  oi 


136  STRUCTURAL    GEOLOGY. 

sediment.  On  an  irregular  coast  like  this,  especially, 
the  gravel,  sand,  and  mud  of  different  colors  and  tex- 
tures, and  the  different  kinds  of  shelly  bottom,  form  a 
patchwork,  the  patches  being,  for  the  most  part,  of 
limited  extent  and  shading  off  gradually  into  each 
other. 

On  a  more  regular  coast,  like  that  of  New  Jersey, 
the  sediments  are  distributed  with  corresponding  uni- 
formity, the  changes  are  less  frequent  and  more  grad- 
ual, and  we  have  here  a  better  chance  to  observe  the 
normal  arrangement  of  the  sediments  along  a  line  from 
the  shore  seawards  —  gravel,  sand,  mud,  and  shells. 
On  the  beach  we  find  the  shingle  and  coarse  pebbles, 
shading  off  rapidly  into  fine  pebbles  and  sand.  The 
zone  or  belt  of  sandy  bottom  may  vary  in  width  from 
a  mile  or  two  to  twenty  miles  or  more,  becoming  grad- 
ually finer  and  changing  into  clay  or  mud,  which 
covers,  usually,  a  much  broader  zone,  sometimes  ex- 
tending into  the  deeper  parts  of  the  sea,  but  gradually 
giving  way  to  calcareous  sediments.  Hence  we  may 
say  that  the  finer  the  sediment  the  greater  the  area 
over  which  it  is  spread;  but,  on  the  other  hand,  the 
coarser  the  sediment  the  more  rapidly  it  increases  in 
thickness.  In  other  words,  the  horizontal  extent  of  a 
formation  deposited  in  any  given  period  of  time  is 
inversely,  and  the  vertical  extent  or  thickness  is  directly, 
proportional  to  the  size  of  the  particles. 

Observations  made  in  deep  wells  and  mines,  and 
where,  by  upturning  and  erosion,  the  edges  of  the 
strata  are  exposed  on  the  surface,  show  that  the  verti- 
cal order  of  the  different  kinds  of  sedimentary  rocks 
in  the  earth's  crust  is  extremely  variable.  But  when 


PETROLOGY.  137 

we  take  a  general  view  of  a  great  formation,  it  is  often 
apparent  that  it  consists  chiefly  of  coarse-grained  rocks 
in  the  lower  part  and  fine-grained  rocks  in  the  upper 
part.  This  is,  in  general,  a  necessary  consequence  of 
the  fact  that  a  great  thickness  of  sediments  can  only 
be  formed  on  a  subsiding  sea-floor.  Such  a  formation 
must  consist  chiefly  of  shore  deposits,  and  be  deposited 
near  the  shore  where  the  sea  is  shallow.  Hence,  10,000 
feet  of  sediments  implies  nearly  that  amount  of  subsi- 
dence. In  consequence,  the  shore  line  and  the  several 
zones  of  sediment  advance  towards  the  land ;  and  sand 
is  deposited  where  gravel  was  at  first,  and  as  the  sub- 
sidence continues,  both  clay  and  limestone  are  finally 


Fig.  4.  —  Overlap  and  unconformability. 

deposited  over  the  original  beach.  When  the  sea- 
floor  rises,  the  order  of  the  sediments  is  reversed ; 
and  it  will  be  observed  that  in  consequence  of  the 
advance  and  retreat  of  the  shore-line,  the  formations 
grow  edgewise  to  a  considerable  extent. 

OVERLAP  AND  INTERPOSITION  OF  STRATA.  —  Another 
consequence  of  the  constant  oscillation  of  the  shore- 
line is  that  successive  deposits  in  the  same  sea  will 
often  cover  different  and  unequal  areas.  When,  in 
consequence  of  subsidence,  one  formation  extends 
beyond  and  covers  the  edge  of  another,  as  shown  in 
Fig.  4,  we  have  the  phenomenon  described  as  overlap. 


130  STRUCTURAL    GEOLOGY. 

Interposition  is  similar,  being  the  case  where  a  for- 
mation (Fig.  5,  <r.)  does  not,  in  certain  directions, 
cover  so  wide  an  area  as  the  strata  (b.  d.)  above 
and  below  it,  which  are  thus  sometimes  found  in  con- 
tact, although  normally  separated  by  the  entire  thick- 
ness of  the  intermediate  and,  seemingly,  interposed 
stratum. 


Fig-  5-  —  Interposition  of  strata. 


UNCONFORMABILITY.  —  We  have  already  seen  that 
the  rocks  on  the  land  are  being  constantly  worn  away 
by  the  agents  of  erosion ;  and  it  is  also  a  matter  of 
common  observation  that  the  strata  thus  exposed  are 
often  not  horizontal,  but  highly  inclined,  having  been 
greatly  disturbed  and  crumpled  during  their  elevation. 
Now,  when  such  a  land-surface  subsides  to  form  the 
sea-bottom,  and  new  strata  are  spread  horizontally 
over  it,  they  will  lie  across  the  upturned  and  eroded 
edges  of  the  older  rocks,  and  fill  the  hollows  worn 
out  of  the  latter,  as  shown  in  Fig.  6 ;  and  the  new 


PETROLOGY.  *39 

formation  is  then  said  to  rest  unconformably  upon  the 
older.  Two  strata  or  formations  are  unconformable 
when  the  older  has  suffered  erosion  (Fig.  6),  or  both 
disturbance  and  erosion  (Fig.  4)  before  the  deposition 
of  the  newer. 

When  strata  are  conformable,  the  deposition  may 
be  presumed  to  have  been  nearly  or  quite  continuous ; 
but  unconformability  clearly  proves  a  prolonged  inter- 
ruption of  the  deposition  during  which  the  elevation, 
erosion,  and  subsidence  of  the  sea-bottom  took  place. 
The  section  in  Fig.  7  shows  a  second  unconformability, 


Fig.  6.  —  Unconformability. 

proving  that  the  sea-bottom  has  here  been  lifted  three 
times  to  form  dry  land.  An  unconformability  may 
sometimes  be  clearly  established  when  the  actual  con- 
tact of  the  two  formations  cannot  be  seen,  as  where 
the  new  formation  is  a  conglomerate  containing  frag- 
ments of  the  older. 

IRREGULARITIES  OF  STRATIFICATION.  —  These  are  es- 
pecially noticeable  in  sandstone  and  conglomerate, 
which  have  been  deposited  chiefly  by  strong,  local,  and 
variable  currents ;  the  kind  and  quantity  of  sediment, 
of  course,  varying  with  the  strength  and  direction  of 
the  current.  Two  kinds  of  irregularity  only  may  be 
specially  noticed  here  :  (i)  contemporaneous  erosioi) 


140 


STRUCTURAL    GEOLOGY. 


and  deposit,  where,  in  consequence  of  a  change  in  the 
currents,  fine  material  recently  deposited  is  partially 
swept  away  and  its  place  taken  by  coarser  sediments  j 
and  (2)  oblique  lamination,  or  current-bedding,  where 
the  strata  are  horizontal  as  usual,  but  the  component 


Fig.  7.  —  Double  Unconformability :  q.  quartzite;  s.  sandstone;  d.  drift. 


laminse  are  inclined  at  various  angles.  This  structure 
is  characteristic  of  sediments  swept  along  by  strong 
currents,  especially  when  deposited  in  shallow  basins 
or  depressions. 


PETROLOGY.  141 

RIPPLE-MARKS.  —  All  who  have  been  on  a  beach  or 
sand-bar  must  have  noticed  the  lines  of  wavy  ridges 
and  hollows,  or  ripples,  on  the  surface  of  the  sand. 
These  are  sand-waves,  produced  by  water  moving  over 
the  sand,  or  by  air  moving  over  dry  sand,  as  ordinary 
waves  are  formed  by  air  moving  over  water.  Each 
tide  usually  effaces  the  ripple-marks  made  by  its  pre- 
decessor and  leaves  a  new  series,  to  be  obliterated 
by  the  next  tide.  But  where  sediment  is  constantly 
accumulating,  a  rippled  surface  may  be  gently  over- 
spread by  a  new  layer,  and  thus  preserved.  Other 
series  of  ripples  may,  in  like  manner,  be  formed  and 
preserved  in  overlying  layers ;  and  when  the  beach 
becomes  a  firm  sandstone,  a  section  of  it  will.show  the 
rippled  surfaces  almost  as  distinctly  as  when  they  were 
first  formed  (Fig.  8).  Ripple-marks  are  most  perfect 
in  fine  sand.  They  are  not  formed  in  gravel,  because 
it  is  too  coarse  ;  nor  in  clay,  because  it  is  too  tenacious. 
They  are  usually  limited  to  shallow  water;  and  are 
always  regarded  as  proving  that  the  rocks  in  which 
they  occur  are  shallow-water  or  beach  deposits.  They 
are  normally  at  right  angles  to  the  current  that  pro- 
duces them,  and  where  this  changes  with  the  direction 
of  the  wind,  cross-ripples  and  other  irregularities  are 
introduced.  Ripple- marks  are  also  usually  parallel 
with  the  beach,  and  when  they  are  found  in  the  rocks 
they  give  us  the  direction,  as  well  as  the  position,  of 
the  ancient  shore-line. 

Again,  the  friction  of  the  water  pushes  the  sand- 
grains  along,  rolling  them  up  on  one  side  of  the  ripple 
and  letting  them  fall  down  on  the  other.  Hence 
ripples,  formed  by  a  current  are  always  moving  and  are 


142  STRUCTURAL    GEOLOGY. 

unsymmetrical  on  the  cross-section,  presenting  a  long, 
gentle  slope  toward  the  current,  and  a  short,  steep  slope 
away  from  it,  the  arrow  in  the  figure  indicating  the 
direction  of  the  current,  or  of  the  sea  in  the  case  of  a 
beach.  And  we  may  thus  learn  from  the  fossil  ripples, 
in  some  cases,  not  only  the  position  and  direction  of 
the  ancient  shore,  but  also  on  which  side  the  land  lay, 
and  on  which  side  the  sea.  When  the  water  is  in  a 
state  of  oscillation,  without  any  distinct  current,  more 
symmetrical  ripples  are  produced. 


Fig.  8.  —  Ripple-marks  in  sandstone. 

RILL-MARKS,  RAIN-PRINTS,  AND  SUN-CRACKS.  —  "  One 
of  the  most  fascinating  parts  of  the  work  of  a  field- 
geologist  consists  in  tracing  the  shores  of  former  seas 
and  lakes,  and  thus  reconstructing  the  geography  of 
successive  geological  periods."  His  conclusions,  as  we 
have  already  seen,  are  based  largely  upon  the  nature 
of  the  sediments ;  but  still  more  convincing  is  the  evi- 
dence afforded  by  those  superficial  features  of  the 
strata,  which,  like  ripple-marks,  seem,  by  themselves, 
quite  insignificant.  And  among  these  he  lays  special 
emphasis  upon  those  which  show  that  during  their 
deposition  strata  have  at  intervals  been  laid  bare  to 
sun  and  air. 

During  ebb  tide  water  which  has  been  left  at  the 


PETROLOGY.  143 

upper  edge  of  the  beach  runs  down  across  the  beach 
in  small  rills,  which  excavate  miniature  channels ;  and 
when  these  are  preserved  in  the  hard  rocks,  they  prove 
that  the  latter  are  beach  deposits,  and,  like  the  ripple- 
marks,  show  the  direction  of  the  old  shore. 

If  a  heavy  shower  of  rain  falls  on  a  muddy  beach  or 
flat,  the  sediment  deposited  by  the  returning  tide  may 
cover,  without  obliterating,  the  small  but  characteristic 
impressions  of  the  individual  drops  ;  and  these  mark- 
ings are  frequently  found  well  preserved  in  the  hardest 
slates  and  sandstones,  testifying  unequivocally  to  the 
conditions  under  which  the  rocks  were  formed.  In 
some  cases  tlie  rain-prints  are  found  to  be  ridged  up 
on  one  side  only,  in  such  a  manner  as  to  indicate  that 
the  drops  as  they  fell  were  driven  aslant  by  the  wind. 
The  prominent  side  of  the  marking,  therefore,  indicates 
the  side  towards  which  the  wind  blew. 

Muddy  sediments,  especially  in  lakes  and  rivers, 
are  often  exposed  to  the  air  and  sun  during  periods  of 
drouth,  and  as  they  gradually  dry  up,  polygonal  cracks 
are  formed.  The  sediment  of  the  next  layer  will  fill 
these  sun-cracks;  and  when,  as  often  happens,  it  is 
slightly  different  from  the  dessicated  layer,  they  may 
still  be  traced.  Sun-cracks  preserved  in  this  way  are 
very  characteristic  of  argillaceous  rocks,  and,  of  course, 
prove  that  in  early  times,  as  at  the  present  day,  sedi- 
ments of  this  class  were  exposed  by  the  temporary 
retreat  of  the  water.  The  foot-prints  or  trails  of  land- 
animals  are  often,  as  in  the  sandstones  and  shales  of 
the  Connecticut  Valley,  associated  with,  and  of  course 
strongly  corroborate,  all  these  other  evidences  of  shore 
deposits.  From  the  foot-prints  preserved  in  the  rocks 


144  STRUCTURAL    GEOLOGY. 

we  pass  naturally  to  the  consideration  of  the  fossil 
remains  of  plants  and  animals  found  entombed  in  the 
strata. 

FOSSILS.  —  Although  fossils  find  their  highest  interest 
in  the  light  which  they  throw  upon  the  succession  of 
life  on  the  globe,  they  may  also  be  properly  regarded 
as  structural  features  of  stratified  rocks  ;  and  any  one 
who  has  seen  the  dead  shells,  crabs,  fishes,  etc.,  on 
the  beach  will  readily  understand  how  fossils  get  into 
the  rocks.  It  is  not  our  province  here  to  study  the 
structure  of  the  fossils  themselves,  for  that  would  in- 
volve us  in  a  course  in  paleontology,  a  task  belonging 
to  the  biologist  rather  than  the  geologist ;  but  we  will 
merely  observe  the  three  principal  degrees  in  the 
preservation  of  fossils  :  — 

1 .  Original  composition  not  completely  changed.  — 
Extinct  elephants  have  been  found  frozen  in  the  river- 
bluffs  of  Siberia  so  perfectly  preserved  that  dogs  and 
wolves  ate  their  flesh.     The  bodies  of  animals  are  also 
found  well  preserved  in  peat-bogs.     All  coal  is  simply 
fossil  vegetation  retaining  in  a  large  degree  the  original 
composition ;  and  the  same  is  true  of  ferns,  etc.,  pre- 
served as  black  impressions  in  the  rocks.     All  bones 
and  shells  consist  of  mineral  matter  which  makes  them 
hard,  and  animal  matter  which  makes  them  tough  and 
strong.     In  very  many  cases,  especially  in  the  newer 
formations,  the  animal  matter  is  still  partially,  and  the 
mineral  matter  almost  wholly,  intact. 

2.  Original  composition  completely  changed,  but  form 
and  structure  preserved.  — All  kinds  of  fossils  are  com- 
monly called  petrifactions,  but  only  those  preserved  in 
this  second  way  are  truly  petrified,  i.e.,  turned  to  stone. 


PETROLOGY.  145 

"  Petrified  wood  is  the  best  illustration,  and  in  a  good 
specimen  not  only  the  external  form  of  the  wood,  not 
only  its  general  structure —  bark,  wood,  radiating  silver- 
grain,  and  concentric  rings  of  growth  —  are  discernible, 
but  even  the  microscopic  cellular  structure  of  the  wood, 
and  the  exquisite  sculpturing  of  the  cell-walls,  are  per- 
fectly preserved,  so  that  the  kind  of  wood  may  often 
be  determined  by  the  microscope  with  the  utmost 
certainty.  Yet  not  one  particle  of  the  organic  matter 
of  the  wood  remains.  It  has  been  entirely  replaced 
by  mineral  matter;  usually  by  some  form  of  silica. 
The  same  is  true  of  the  shells  and  bones  of  animals."  — 
LE  CONTE. 

3.  Original  composition  and  structure  both  obliter- 
ated, and  form  alone  preserved.  —  This  occurs  most 
commonly  with  shells,  although  fossil  trees  are  also 
often  good  illustrations.  The  general  result  is  accom- 
plished in  several  ways :  (a)  The  shell  after  being 
buried  in  the  sediment  may  be  removed  by  solution, 
leaving  a  mould  of  its  external  form,  (b)  This  'mould 
may  subsequently  be  filled  by  the  infiltration  of  finer 
sediment,  forming  a  cast  of  the  exterior  of  the  shell. 
(c)  The  shell,  before  its  solution,  may  have  been  filled 
with  mud ;  and  if  the  shell  itself  is  then  dissolved,  we 
have  a  cast  of  its  interior  in  a  mould  of  its  exterior. 

TIME   REQUIRED    FOR  THE    FORMATION   OF   STRATIFIED 

ROCKS.  —  Many  attempts  have  been  made  to  deter- 
mine the  time  required  for  the  deposition  of  any  given 
thickness  of  stratified  rocks.  Of  course,  only  roughly 
approximate  results  can  be  hoped  for  in  most  cases ; 
but  these  are  at  least  sufficient  to  make  it  certain  that 
geological  time  ig  very  long.  The  average  relative  rate 


146  STRUCTURAL    GEOLOGY. 

of  growth  of  different  kinds  of  sediment  is,  however, 
less  open  to  doubt,  for  we  have  already  seen  that 
coarse  sediments  like  gravel  and  sand  accumulate 
much  more  rapidly  than  finer  sediments  like  clay  and 
limestone ;  and  we  are  sometimes  able  to  compare 
these  two  classes  of  rocks  on  a  very  large  scale. 

Thus,  during  what  is  known  as  the  Paleozoic  era,  a 
sea  extended  from  the  Blue  Ridge  to  the  Rocky  Moun- 
tains. Along  the  eastern  margin  of  this  sea,  where  the 
Alleghany  Mountains  now  stand,  sediments  —  chiefly 
conglomerate  and  sandstone,  with  some  slate  and  less 
limestone  —  accumulated  to  a  thickness  of  nearly 
40,000  feet.  Toward  the  west,  away  from  the  old 
shore-line,  the  coarse  sediments  gradually  die  out,  and 
the  formations  become  finer  and  thinner.  In  western 
Ohio  and  Indiana,  slate  and  limestone  predominate ; 
while  in  the  central  part  of  the  ancient  sea,  in  Illi- 
nois and  Missouri,  the  paleozoic  sediments  are  almost 
wholly  limestones,  and  have  a  thickness  of  only  4,000 
to  5,000  feet.  In  other  words,  while  one  foot  of  lime- 
stone was  forming  in  the  Mississippi  Valley,  eight  to 
ten  feet  of  coarser  sediments  were  deposited  in  Penn- 
sylvania. 

The  best  estimates  show  that  coral-reefs  rise  —  i.e., 
limestones  are  formed  on  them  —  at  the  rate  of  about 
one  foot  in  two  hundred  years.  But  coral  limestones 
grow  much  more  rapidly  than  limestones  in  general. 
Sandstones  sometimes  accumulate  so  rapidly  that  trees 
are  buried  before  they  have  time  to  decay  and  fall 
(Fig.  9).  Such  a  buried  forest,  like  a  coal-bed,  rep- 
resents a  land  surface,  and  proves  a  subsidence  of  the 
land ;  and  in  some  cases,  as  indicated  by  the  section, 


PETROLOGY. 


'47 


repeated  oscillations  of  the  crust  may  be  proved  in 
this  way. 

The  mud  deposited  by  the  annual  overflow  of  the 
Nile  is  forty  feet  thick  near  the  ancient  city  of  Mem- 
phis ;  and  the  pedestal  of  the  statue  of  Rameses  II., 
believed  to  have  been  erected  B.C.  1361,  is  buried  to 
a  depth  of  nine  feet,  four  inches,  indicating  that  13,500 
years  have  elapsed  since  the  Nile  began  to  spread 
its  mud  over  the  sands  of  the 
desert. 

But  the  greatest  difficulty  in 
estimating  the  time  required 
for  the  formation  of  any  series 
of  strata  arises  from  the  fact 
that  we  cannot  usually  even 
guess  at  the  length  of  the 
periods  when  the  deposition 
has  been  partially  or  wholly 
interrupted.  Now  and  then, 
however,  we  find  evidence  that 
these  periods  may  be  very  long. 
A  layer  of  fossil  shells  in  sandstone  or  slate  proves  an 
interruption  of  mechanical  deposition.  Beds  of  coal, 
fossil  forests,  and  other  indications  of  land  surfaces 
are  still  more  conclusive.  The  interposition  of  strata 
(Fig.  5)  proves  a  prolonged  interruption  of  deposition 
over  the  area  not  covered  by  the  interposed  bed.  But 
the  most  important  of  all  evidence  is  that  afforded  by 
unconformability ;  and  the  length  of  the  lost  interval 
between  the  two  formations  is  measured  approximately 
by  the  erosion  of  the  older. 


Fig.  9.  —  Erect  fossil  trees. 


148 


STRUCTURAL    GEOLOGY. 


Fig.  10.  —  Typical  dikes. 


Original  Structures  of  Eruptive  Rocks. 

The  structures  of  this  class  are  divisible  into  those 
pertaining  to  the  volcanic  rocks  and  those  pertaining 
to  the  fissure  or  dike  rocks.  But  since  volcanoes  are 
rare  in  this  part  of  the  world,  while  dikes  are  well 

developed  in  many  sec- 
tions of  our  country,  it 
seems  best  to  give  our 
attention  chiefly  to  the 
latter. 

The  term  dike  is  a 
general  name  for  all 
masses  of  eruptive  rocks 
that  have  cooled  and 
solidified  in  fissures  or 
cavities  in  the  earth's 
crust.  But  the  name  is 
commonly  restricted  to 
the  more  regular,  wall- 
like  masses  (Fig.  10), 
those  having  extremely 
irregular  outlines,  like 
most  masses  of  granite 

(Fig.  n),  being  known  simply  as  eruptive  masses. 
The  propriety  of  this  distinction  is  apparent  when  we 
consider  the  origin  of  dike  as  a  geological  term.  It 
was  first  used  in  this  sense  in  southern  Scotland,  where 
almost  any  kind  of  a  wall  or  barrier  is  called  a  dike. 
The  dikes  traverse  the  different  stratified  formations 
like  gigantic  walls,  which  are  often  encountered  by 
the  coal-miners,  and  on  the  surface  are  frequently  left 


Fig.  ii.  —  Section  of  a  granite  mass. 


PETROLOGY.  149 

in  relief  by  the  erosion  of  the  softer  enclosing  rock, 
so  that  in  the  west  of  Scotland,  especially,  they  are 
actually  made  use  of  for  enclosures.  In  other  cases 
the  dike  has  decayed  faster  than  the  enclosing  rock, 
and  its  position  is  marked  by  a  ditch-like  depression. 
The  narrow,  straight,  and  perpendicular  clefts  or  chasms 
observed  on  many  coasts  are  usually  due  to  the  re- 
moval of  the  wall-like  dikes  by  the  action  of  the  waves. 
Dikes  are  sometimes  mere  plates  of  rock,  traceable  for 
a  few  yards  only ;  and  they  range  in  size  from  that  up 
to  those  a  hundred  feet  or  more  in  width,  and  trace- 
able for  scores  of  miles  across  the  country,  their  out- 
crops forming  prominent  ridges.  The  sides  of  dikes 
are  often  as  parallel  and  straight  of  those  of  built  walls, 
the  resemblance  to  human  workmanship  being  height- 
ened by  the  numerous  joints  which,  intersecting  each 
other  along  the  face  of  a  dike,  remind  us  of  well-fitted 
masonry. 

FORMS  OF  DIKES.  —  A  dike  is  essentially  a  casting. 
Melted  rock  is  forced  up  from  the  heated  interior  into 
a  cavity  or  crack  in  the  earth's  crust,  cools  and  solidi- 
fies there,  and,  like  a  metallic  casting,  assumes  the 
form  of  the  fissure  or  mould.  In  other  words,  the 
form  of  the  dike  is  exactly  that  of  the  fissure  into 
which  the  lava  was  injected.  Now  the  forms  of  fis- 
sures depend  partly  upon  the  nature  of  the  force  that 
produces  them,  but  very  largely  upon  the  structure 
—  and  especially  the  joint-structure  —  of  the  enclos- 
ing rocks.  Nearly  all  rocks  are  traversed  by  planes 
of  division  or  cracks  called  joints,  which  usually  run 
in  several  directions,  dividing  the  rock  into  blocks. 
And  it  is  probable  that  dike-fissures  are  most  com' 


r5°  STRUCTURAL    GEOLOGY. 

monly  produced,  not  by  breaking  the  rocks  anew,  but 
by  widening  or  opening  the  pre-existing  joint-cracks. 
Hence  the  straight  and  regular  jointing  of  slate,  lime- 
stone and  most  sedimentary  rocks  is  accompanied  by 
wall-like  dikes  —  the  typical  dikes  (Fig.  10)  ;  while 
the  more  irregular  jointing  of  granite  and  other  mas- 
sive rocks  gives  rise  to  sinuous,  branching,  variable 
dikes.  The  general  dependence  of  dikes  upon  the 
joint-structure  of  the  rocks  is  proved  by  the  facts  that 

dikes,  like  joints,  are 
normally  vertical  or 
highly  inclined,  and 
that  they  are  usually 
parallel  with  the  prin- 
cipal systems  of  joints 
in  the  same  district. 

Fig.  ».  -  Dike  with  regular  brariches.        The  Wall-like  dikes  also 

give  off  branches,  but 
•usually  in  a  regular  manner,  as  shown  in  Fig.  12. 

STRUCTURE  OF  DIKES.  —  The  rock  traversed  by  a 
dike  is  called  the  country  or  wall  rock.  Fragments 
of  this  are  often  torn  off  by  the  igneous  material,  and 
become  enclosed  in  the  latter.  Such  enclosed  frag- 
ments may  sometimes  form  the  main  part  of  the  dike, 
which  then,  since  they  are  necessarily  angular,  often 
assumes  the  aspect  of  a  breccia.  This  is  the  only  im- 
portant exception  to  the  rule  that  dikes  are  homoge- 
neous in  composition ;  /.<?.,  in  the  same  dike  we  can 
usually  find  —  from  end  to  end,  from  side  to  side,  and 
probably  from  top  to  bottom  —  no  essential  difference 
in  composition.  But  there  is  often  a  marked  contrast 
in  texture  between  different  parts  of  a  dike,  and  espe« 


PETROLOGY.  151 

dally  between  the  sides  and  central  portion.  The 
liquid  rock  loses  heat  most  rapidly  where  it  is  in  con- 
tact with  the  cold  walls  of  the  fissure,  and  solidifies 
before  it  has  time  to  crystallize,  remaining  compact 
and  sometimes  even  glassy;  while  in  the  middle  of 
the  dike,  unless  it  is  very  narrow,  it  cools  so  slowly  as 
to  develop  a  distinctly  crystalline  texture.  There  is 
no  abrupt  change  in  texture,  but  a  gradual  passage 
from  the  compact  border  to  the  coarsely  crystalline  or 
porphyritic  middle  portion.  It  is  obvious  that  a  simi- 
lar gradation  in  texture  must  exist  between  the  top 
and  bottom  of  a  dike. 

CONTACT  PHENOMENA. — Under  this  head  are  grouped 
the  interesting  and  important  phenomena  observable 
along  the  contact  between  the  dike  and  wall-rock. 
These  throw  light  upon  the  conditions  of  formation  of 
dikes,  and  are  often  depended  upon  to  show  whether 
a  rock  mass  is  a  dike  or  not.  The  student  will  observe 
here  :  — 

1.  The  detailed  form  of  the  contact.     It  may  be 
straight  and  simple,  or  exceedingly  irregular,  the  dike 
penetrating  the  wall,  and  enclosing  fragments  of  it,  as 
in  Fig.  n,  which  is  a  typically  igneous  contact. 

2.  The  alteration  of  the  wall-rock  by  heat.     This 
may  consist  in  :   (a)  coloration,  shales  and  sandstones 
being  reddened  in  the  same  way  as  when  clay  is  burnt 
for   bricks ;    (ft)    baking  and   induration,   sandstone 
being  converted  into  quartzite  and  even  jasper ;  clay, 
slate,  etc.,  being  not  only  baked  to  a  flinty  hardness, 
but  actually  vitrified,  as  in  porcelainite ;  and  bitumi- 
nous coal  being  converted  into  natural  coke  or  anthra- 
cite; and  (c)  crystallization,  chalk,  and  other  lime- 


152  STRUCTURAL    GEOLOGY. 

stones  being  changed  to  marble,  and  crystals  of  pyrite, 
calcite,  quartz,  etc.,  being  developed  in  slate,  sand- 
stone, and  other  rocks. 

3.  The  alteration  of  the  dike-rock  by  (a)  more 
rapid  cooling,  and  (b)  the  access  of  thermal  waters. 

The  alteration  of  the  wall-rock  may  extend  only  a 
few  inches  or  many  yards  from  the  dike,  gradually 
diminishing  with  the  distance ;  and  the  cases  are  sur- 
prisingly numerous  where  there  is  no  perceptible  alter- 
ation ;  and,  again,  the  alteration  is  usually  mutual,  the 
dike-rock  being  altered  in  texture,  color,  and  compo- 
sition. 


Fig.  13.  —  Ideal  cross-section  of  Fig.  14.  —  Ideal  cross-section  of 

a  laccolite.  a  volcano. 


INTRUSIVE  BEDS.  —  We  commonly  think  of  dikes 
as  cutting  across  the  strata,  but  they  often  lie  in  planes 
parallel  with  them  ;  and  the  same  dike  may  run  across 
the  beds  in  some  parts  of  its  course  and  between  them 
in  others  (Fig.  12),  or  the  conformable  dike  may  be 
simply  a  lateral  branch  of  a  main  vertical  dike,  as 
shown  in  the  same  figure.  All  dikes  or  portions  of 
dikes  lying  conformably  between  the  strata  are  called 
intrusive  beds  or  sheets. 

When  a  dike  fails  to  reach  the  surface,  but  spreads 
out  horizontally  between  the  strata,  forming  a  thick 
dome  or  oven-shaped  intrusive  bed,  the  latter  is  called 


PETROLOGY.  153 

a  laccolite  (Fig.  13).  Laccolites  are  sometimes  of 
immense  volume,  containing  several  cubic  miles  of 
rock.  Fig.  14  enables  us  to  compare  the  laccolite 
with  the  volcano. 

In  the  one  case  a  large  mound  of  eruptive  material 
accumulates  between  the  strata,  the  overlying  beds 
being  lifted  into  a  dome ;  while  in  the  other  case  the 
fissure  or  vent  reaches  the  surface,  and  the  mound  of 
lava  is  built  up  on  top  of  the  ground. 

COTEMPORANEOUS  BEDS.  —  When  the  lava  emitted 
by  a  crater  is  sufficiently  liquid,  it  spreads  out  hori- 
zontally, forming  a  volcanic  sheet  or  bed.  If  such  an 
eruption  is  submarine,  or  the  lava  flow  is  subsequently 
covered  by  the  sea,  sedimentary  deposits  are  formed 
over  it ;  and  beds  of  lava  which  thus  come  to  lie  con- 
formably between  sedimentary  strata  are  known  as 
cotemporaneous  sheets  or  beds,  because  they  belong, 
in  order  of  time,  in  the  position  in  which  we  find 
them,  being,  like  any  member  of  a  stratified  series, 
newer  than  the  underlying  and  older  than  the  over- 
lying strata.  Cotemporaneous  lava-flows  are  some- 
times repeated  again  and  again  in  the  same  district, 
and  thus  important  formations  are  built  up  of  alternat- 
ing igneous  and  aqueous  deposits.  Evidently,  the 
student  who  would  read  correctly  the  record  of  igne- 
ous activity  in  the  past  must  be  able  to  distinguish 
intrusive  and  cotemporaneous  beds.  The  principal 
points  to  be  considered  in  making  this  distinction 
are  :  (i)  The  intrusive  bed  is  essentially  a  dike,  dense 
and  more  or  less  crystalline  in  texture,  altering,  and 
often  enclosing  fragments  of,  both  the  underlying  and 
overlying  strata,  and  frequently  jogging  across  or  pene* 


154  STRUCTURAL    GEOLOGY. 

trating  the  sediments.  (2)  The  cotemporaneous  bed, 
on  the  other  hand,  being  essentially  a  lava-flow,  is 
much  less  dense  and  crystalline,  being  usually  distinctly 
scoriaceous  or  amygdaloidal,  especially  at  the  borders, 
and  the  underlying  strata  alone  showing  heat  action, 
or  occurring  as  enclosures  in  the  lava ;  for  the  overly- 
ing strata  are  newer  than  the  lava,  and  often  consist 
largely,  at  the  base,  of  water- worn  fragments  of  the  lava. 

AGES  OF  DIKES.  —  The  ages  of  dikes  may  be  esti- 
mated in  several  ways.  They  are  necessarily  newer 
than  any  stratified  formation  which  they  intersect  or 
of  which  they  enclose  fragments ;  but  any  formation 
crossing  the  top  of  a  dike  must  usually  be  regarded  as 
newer  than  the  dike,  especially  if  it  contains  water- 
worn  fragments  of  the  dike  rock. 

The  relative  ages  of  different  dikes  are  determined 
by  their  relations  to  the  stratified  formations ;  and  still 
more  easily  by  their  mutual  intersections,  on  the  prin- 
ciple that  when  two  dikes  cross  each  other,  the  inter- 
secting must  be  newer  than  the  intersected  dike.  It 
is  sometimes  possible,  in  this  way,  to  prove  several 
distinct  periods  of  eruption  in  the  same  limited  dis- 
trict. The  textures  of  dikes  also  often  afford  reliable 
indications  of  their  ages  ;  for,  as  we  have  already  seen, 
the  upper  part  of  a  dike,  cooling  rapidly  and  under 
little  pressure,  must  be  less  dense  and  crystalline  than 
the  deep-seated  portion,  which  cools  slowly  and  under 
great  pressure. 

Now,  the  lower,  coarsely  crystalline  part  of  a  dike 
can  usually  be  exposed  on  the  surface  only  as  the 
result  of  enormous  erosion ;  and  erosion  is  a  slow 
process,  requiring  vast  periods  of  time.  Hence,  when 


PETROLOGY.  155 

we  see  a  coarse-grained  dike  outcropping  on  the  sur- 
face, we  are  justified  in  regarding  it  as  very  old,  for 
all  the  fine-grained  upper  part  has  been  gradually  worn 
away  by  the  action  of  the  rain,  frost,  etc.  Other  things 
being  equal,  coarse-grained  must  be  older  than  fine- 
grained dikes ;  and  the  texture  of  a  dike  is  at  once  a 
measure  of  its  age  and  of  the  amount  of  erosion  which 
the  region  has  suffered  since  it  was  formed. 

ERUPTIVE  MASSES.  —  In  striking  contrast  with  the 
more  or  less  wall-like  dikes  are  the  highly  irregular, 
and  even  ragged,  outlines  of  the  eruptive  masses  ;  and 
it  is  worth  while  to  notice  the  probable  cause  of  this 
contrast.  The  true  dikes  are  formed,  for  the  most 
part,  of  comparatively  fine-grained  rocks  —  the  typical 
"  traps  "  ;  while  the  eruptive  masses  consist  chiefly  of 
the  coarse-grained  or  granitic  varieties.  Now  we  have 
just  seen  that  the  coarse-grained  rocks  have  been 
formed  at  great  depths  in  the  earth's  crust,  while  the 
fine-grained  are  comparatively  superficial.  But  we 
have  good  reason  for  believing  that  the  joint-struc- 
ture, upon  which  the  forms  of  dikes  so  largely  de- 
pend, is  not  well  developed  at  great  depths,  where 
the  rocks  are  toughened,  if  not  softened,  by  the  high 
temperature.  In  other  words,  trap  dikes  are  formed 
in  the  jointed  formations,  which  break  regularly ;  while 
the  granitic  masses  are  formed  where  the  absence  of 
joint-structure  and  a  high  temperature  combine  to 
cause  extremely  irregular  rifts  and  cavities  when  the 
crust  is  broken. 

VOLCANIC  PIPES  OR  NECKS.  —  Every  volcano  and 
every  lava -flow  or  volcanic  sheet  must  be  connected 
with  the  earth's  interior  by  a  channel  or  fissure,  which 


156  STRUCTURAL    GEOLOGY. 

becomes  a  dike  when  the  lava  ceases  to  flow.  But 
the  converse  proposition  is  not  true,  for  it  is  probable 
that  many  dikes  did  not  originally  reach  the  surface, 
but  have  been  exposed  by  subsequent  denudation. 
This  is  conspicuously  the  case  with  laccolites  and 
other  forms  of  intrusive  sheets.  Volcanic  sheets  or 
beds  have  probably  often  resulted  from  the  overflow 
of  the  lava  at  all  points  of  an  extensive  fissure  or  sys- 
tem of  fissures  ;  but  the  vent  of  the  true  volcano  must 
be  more  circumscribed,  an  approximately  circular  open- 
ing in  the  earth's  crust,  although  doubtless  originating 
in  a  fissure  or  at  the  intersection  of  two  or  more  fis- 
sures, the  lava  continuing  to  flow  at  the  widest  part  of 
the  wound  in  the  crust  long  after  it  has  congealed  in 
the  narrower  parts.  Such  a  tube  is  known  as  the  neck 
or  pipe  of  the  volcano ;  and  volcanic  necks  are  a 
highly  interesting  class  of  dikes,  since  they  determine 
the  exact  location  of  many  an  ancient  volcano,  where 
the  volcanic  pile  itself  has  long  since  been  swept  away. 
Necks  and  dikes  are  the  downward  prolongations  or 
roots  of  the  volcanic  cone  or  sheet,  and  cannot  be 
exposed  on  the  surface  until  the  volcanic  fires  have 
gone  out  and  the  agents  of  erosion  have  removed  the 
greater  part  of  the  ejected  materials. 

Hence,  equally  with  the  dikes  which  originally  failed 
to  reach  the  surface,  they,  wherever  open  to  our  obser- 
vation, testify  to  extensive  erosion  and  a  vast  antiquity. 

Original  Structures  of  Vein  Hocks. 

Many  things  called  veins  are  improperly  so  called, 
such  as  dikes  of  granite  and  trap,  and  beds  of  coal 
and  iron-ore.  The  smaller,  more  irregular,  branching 


PETROLOGY.  157 

dikes,  especially,  are  very  commonly  called  veins,  and 
to  distinguish  the  true  veins  from  these  eruptive  masses, 
they  are  designated  as  mineral  veins  or  lodes,  although 
the  term  lode  is  usually  restricted  to  the  metalliferous 
veins. 

ORIGIN  OF  VEINS.  —  Various  theories  of  the  forma- 
tion of  veins  have  been  proposed,  but  the  most  of 
these  are  of  historic  interest  merely,  for  geologists  are 
now  well  agreed  that  nearly  all  true  veins  have  been 
formed  by  the  deposition  of  minerals  from  solution  in 
fissures  or  cavities  in  the  earth's  crust.  In  many  cases, 
especially  where  the  veins  are  of  limited  extent,  it 
seems  probable  that  a  part  or  all  of  the  mineral  matter 
was  derived  from  the  immediately  enclosing  rocks, 
being  dissolved  out  by  percolating  water;  and  these 
are  known  as  segregation  or  lateral  secretion  veins. 
But  it  is  quite  certain  that  as  a  general  rule  the  min- 
eral solutions  have  come  chiefly  from  below,  the  deep- 
seated  thermal  waters  welling  up  through  any  channel 
opened  to  them,  and  gradually  depositing  the  dis- 
solved minerals  on  the  walls  of  the  fissure  as  the  tem- 
perature and  pressure  are  diminished.  This  case, 
however,  differs  from  the  first  only  in  deriving  the 
vein-forming  minerals  from  more  remote  and  deeper 
portions  of  the  enclosing  rocks  ;  and  thus  we  see  that 
vein-formation,  whether  on  a  large  or  a  small  scale,  is 
always  essentially  a  process  of  segregation. 

We  know  that  every  volcano  and  every  lava  flow 
must  be  connected  below  the  surface  with  a  dike  ;  and 
it  is  almost  equally  certain  that  the  waters  of  mineral 
springs  forming  tufaceous  mineral  deposits  on  the  sur- 
face, as  in  the  geyser  districts,  also  deposit  a  portion 


158  STRUCTURAL    GEOLOGY. 

of  the  dissolved  minerals  on  the  walls  of  the  subter- 
ranean channels,  which  are  thus  being  gradually  filled 
up  and  converted  into  mineral  veins,  which  will  be 
exposed  on  the  surface  when  erosion  has  removed  the 
tufaceous  overflow.  This  connection  of  vein-forma- 
tion with  the  superficial  deposits  of  existing  springs 
has  been  clearly  proved  in  several  important  instances 
in  Nevada  and  California. 

Veins  occur  chiefly  in  old,  metamorphic,  and  highly 
disturbed  formations,  where  there  is  abundant  evidence 
of  the  former  existence  of  profound  fissures,  and  in 
regions  similar  to  those  in  which  thermal  springs  occur 
to-day. 

In  the  supplement  to  the  lithological  section  the 
student  will  find  the  formation  of  a  typical  vein  briefly 
described  and  contrasted  with  that  of  a  typical  dike ; 
also  a  brief  account  of  the  lithological  peculiarities  of 
vein  rocks,  and  general  statements  concerning  their 
relative  abundance  and  vast  economic  importance. 

EXTERNAL  CHARACTERISTICS  OF  VEINS.  — The  typical 
vein  may  be  described  as  a  fissure  of  indefinite  length 
and  depth,  filled  with  mineral  substances  deposited 
from  solution.  Externally,  it  is  very  similar  to  the 
typical  dike,  for  the  fissures  are  made  in  the  same 
way  for  both.  Veins  are  normally  highly  inclined  to 
the  horizon ;  they  exhibit  in  nearly  every  respect  the 
same  general  relations  to  the  structure  of  the  country 
rock  as  dikes ;  and  the  ages  of  veins  are  determined 
in  the  same  way  as  the  ages  of  dikes. 

The  extensive  mining  operations  to  which  veins 
have  been  subjected  in  all  parts  of  the  world,  have 
made  our  knowledge  of  their  forms  below  the  surface 


PETROLOGY.  .  159 

very  full  and  accurate.  It  has  been  learned  in  this 
way  that  very  often  the  corresponding  portions  of  the 
walls  of  a  vein  do  not  coincide  in  position,  but  one 
side  is  higher  or  lower  than  the  other,  showing  that 
the  walls  slipped  over  each  other  when  the  fissure  was 
formed  or  subsequently  j  and  this  faulting  or  displace- 
ment of  the  walls  appears  •  to  be  much  more  common 
with  veins  than  with  dikes,  perhaps  because  the  fis- 
sures remained  open  much  longer.  This  slipping  of 
the  walls  is  the  principal  cause  of  the  almost  constant 
changes  in  the  width  of  veins.  For,  since  the  walls 
are  never  true  planes,  and  are  often  highly  irregular 
any  unequal  movements  must  bring  them  nearer  to- 
gether at  some  points  than  at  others.  As  a  rule,  the 
enormous  friction  accompanying  the  faulting,  either 
crushes  the  wall-rock,  or  polishes  and  striates  it,  pro- 
ducing the  highly  characteristic  surfaces  known  as 
slit, ken-sides.  Where  the  wall  is  finely  pulverized  in 
this  way,  or  is  partially  decomposed  before  or  after 
the  filling  of  the  fissure,  a  thin  layer  of  soft,  argillaceous 
material  is  formed,  separating  the  vein  proper  from 
the  wall-rock.  The  miners  call  this  the  selvage  ;  and 
it  is  a  very  characteristic  feature  of  the  true  fissure 
veins. 

Fragments  of  the  wall-rock  are  frequently  enclosed 
in  veins,  and  the  latter  sometimes  branch  or  divide  in 
such  a  way  as  to  surround  a  large  mass  of  the  wall, 
which  is  known  as  a  "horse."  A  similar  result  is 
accomplished  when  a  fissure  is  re-opened  after  being 
filled,  if  the  new  fissure  does  not  coincide  exactly  with 
the  old.  It  has  been  proved  that  veins  have  thus  been 
re-opened  and  filled  several  times  in  succession ;  and 


160  STRUCTURAL    GEOLOGY, 

in  this  way  fragments  of  the  older  vein  material  be- 
come enclosed  in  the  newer. 

Although  usually  determined  in  direction  by  the 
joint-structure  of  the  country  rock,  veins  are  often 
parallel  with  the  bedding,  especially  in  highly  inclined,, 
schistose  formations.  Such  interbedded  veins  are  com- 
monly distinctly  lenticular  in  form,  occupying  rifts  in 
the  strata  which  thin  out  in  all  directions  and  are  often 
very  limited  in  extent. 

Whether  conforming  with  the  joint-structure  or  bed- 
ding, veins  are  commonly  arranged  in  systems  by  their 
parallelism,  those  of  different  systems  or  directions 
usually  differing  in  age  and  composition,  and  the  older 
veins  being  generally  faulted  or  displaced  when  inter- 
sected by  the  newer. 

INTERNAL  CHARACTERISTICS  OF  VEINS.  —  Internally, 
veins  and  dikes  are  strongly  contrasted  •  and  it  is  upon 
the  internal  features,  chiefly,  as  previously  explained, 
that  we  must  depend  for  their  distinction.  In  met- 
alliferous veins  the  minerals  containing  the  metal 
sought  for  (the  galenite,  sphalerite,  etc.)  are  the  ore ; 
while  the  non- metalliferous  minerals  (the  quartz,  feld- 
spar, calcite,  etc.)  are  called  the  gangue  or  vein-stone 
proper.  Although  the  combinations  of  minerals  in 
veins  are  almost  endless,  yet  certain  associations  of 
ores  with  each  other  and  with  different  gangue  min- 
erals are  tolerably  constant,  and  constitute  an  im- 
portant subject  for  the  student  of  metallurgy  and 
mining. 

When  a  vein  is  composed  of  a  single  mineral,  as 
ouartz,  it  may  rival  a  dike  in  its  homogeneity.  Most 
important  veins,  however,  are  composed  of  several  or  a 


PETROLOGY.  I6l 

large  number  of  minerals,  which  may  be  sometimes 
more  or  less  uniformly  mixed  with  each  other,  but  are 
usually  distributed  in  the  fissure  in  a  very  irregular 
manner.  The  great  granite  veins  which  are  worked 
for  mica,  feldspar  and  quartz,  are  good  illustrations, 
on  a  large  scale,  of  the  structure  of  veins  in  which  sev- 
eral minerals  have  been  deposited  cotemporaneously. 
The  individual  minerals  are  found  to  a  large  extent, 
in  great,  irregular  masses,  with  no  order  observable  in 
their  arrangement. 

When  a  mineral  is  deposited  from  solution,  it  crys- 
tallizes by  preference  on  a  surface  of  similar  composi- 
tion, thus  quartz  on  quartz,  feldspar  on  feldspar,  and 
so  on ;  and  it  seems  probable  that  this  selective  action 
of  the  wall-rock  may  be  a  principal  cause  of  the  irreg- 
ular distribution  of  minerals  in  veins.  It  has  often 
been  observed  in  metalliferous  veins  that  the  richness 
varies  with  the  nature  of  the  adjacent  country  rock. 
This  dependence  of  the  contents  of  a  fissure  upon  the 
wall-rock  may  be  due  in  part  to  the  selective  deposi- 
tion of  the  minerals,  and  in  part  to  their  derivation 
from  the  contiguous  portions  of  the  country  or  wall- 
rock,  as  in  the  so-called  segregated  veins.  Temper- 
ature and  pressure  exert  an  important  influence  upon 
chemical  precipitation,  and  it  is,  therefore,  probable 
that  the  composition  of  many  veins  varies  with  the 
depth. 

Frequently,  perhaps  usually,  the  minerals  of  com- 
posite veins  are  deposited  in  succession,  instead  of  co- 
temporaneously, giving  rise  to  the  remarkable  banded 
structure  so  characteristic  of  this  class  of  veins.  The 
first  mineral  deposited  in  the  fissure  forms  a  layer 


l62 


STRUCTURAL    GEOLOGY. 


covering  each  wall,  and  is  in  turn  covered  by  layers  of 
the  second  mineral,  and  that  by  the  third,  and  so  on, 
until  the  fissure  is  filled,  or  the  solution  exhausted. 
The  distinguishing  features  of  this  structure  are  shown 
in  Fig.  15,  in  which  w  w  represents  the  wall-rock,  a  a, 
b  b,  c  c  are  successive  layers  of  quartz,  fluorite  and 
barite,  and  the  central  band,  d,  is  galenite.  Since  the 
vein  grows  from  the  outside  inward,  the  outer  layers 
are  the  oldest,  and  the  central  layers  are  the  newest ; 
again,  the  layers  are  symmetrically  arranged,  being 

repeated  in  the  reverse 
order  on  opposite  sides 
of  the  middle  of  the  vein  ; 
and,  lastly,  in  layers  com- 
posed of  prismatic  crys- 
tals, as  quartz  (see  the 
figure)  ;  the  crystals  are 
perpendicular  to  the  wall 
and  often  project  into, 
and  even  through,  the 
succeeding  layers.  Such 

a  crystalline  layer  is  called  a  " comb"  and  the  inter- 
locking of  the  layers  in  this  way  is  described  as  the 
comb-structure  of  the  vein.  The  banding  of  veins  is 
thus  strongly  contrasted  with  stratification,  and  with 
the  structure  in  dikes  due  to  the  more  rapid  cooling 
along  the  walls.  The  duplicate  layers  are  often  dis- 
continuous and  of  unequal  thickness,  on  account  of 
the  strong  tendency  to  segregation  in  the  materials. 
This  is  clearly  shown  in  Fig.  16,  drawn  on  a  reduced 
scale  from  a  polished  section  of  a  lead  vein  in  Cum- 
berland, England,  contained  in  the  Museum  of  the 


Fig.  15.  —  Ideal  section  of  a  vein. 


PETROLOGY.  163 

Boston  Society  of  Natural  History.  In  this  the  gangue 
minerals  are  fluorite  (/)  and  barite  (^).  The  central 
band  (fg)  is  a  darker  fluorite  containing  irregular 
masses  of  galenite.  The  banded  structure  of  veins 
is  exactly  reproduced  in  miniature  in  the  banding 
of  agates,  geodes,  and  the  amygdules  formed  in  old 
lavas.  Unfilled  cavities  frequently  remain  along  the 
middle  of  the  vein.  When  small,  these  are  known 


Fig.  16.  —  Section  of  a  lead  vein,  one-fifth  natural  size. 

as  "  pockets."  They  are  commonly  lined  with  crys- 
tals ;  and  when  the  latter  are  minute,  the  pockets  are 
called  druses.  In  metalliferous  veins,  the  ore  is  much 
more  abundant  in  some  parts  than  in  others,  and  these 
ore-bodies,  especially  when  somewhat  definite  in  out- 
line, are  known  in  their  different  forms  and  in  different 
localities,  as  courses,  slants,  shoots,  chimneys,  and 
bonanzas  of  ore.  The  intersections  and  junctions  of 
veins  are  often  among  the  richest  parts,  as  if  the  meet- 
ing of  dissimilar  solutions  had  determined  the  precipi- 
tation of  the  ore. 


1 64  STRUCTURAL    GEOLOGY. 

Metalliferous  veins,  especially,  are  usually  deeply 
decomposed  along  the  outcrop  by  the  action  of  at- 
mospheric agencies.  The  ore  is  oxidized,  and  to  a 
large  extent  removed  by  solution,  leaving  the  quartz 
and  other  gangue  minerals  in  a  porous  state,  stained 
by  oxides  of  iron,  copper,  and  other  metals,  forming 
the  gossan  or  blossom-rock  of  the  vein. 

PECULIAR  TYPES  OF  VEINS.  —  In  calcareous  or 
limestone  formations,  especially,  the  joint-cracks  and 
bedding-cracks  are  often  widened  through  the  solu- 
tion of  the  rock  by  infiltrating  water,  and  thus  become 
the  channels  of  a  more  or  less  extensive  subterranean 
drainage,  by  which  they  are  more  rapidly  enlarged  to 
a  system  of  galleries  and  chambers,  and,  in  some  cases, 
large  limestone  caverns.  The  water  dripping  into  the 
cavern  from  the  overlying  limestone  is  highly  charged 
with  carbonate  of  lime,  which  is  largely  deposited  on 
the  ceiling  and  floor  of  the  cavern,  forming  stalactitic 
and  stalagmitic  deposits.  These  are  masses  of  mineral 
matter  deposited  from  solution  in  cavities  in  the 
earth's  crust,  and  are  essentially  vein-formations. 
Portions  of  caverns  deserted  by  the  flowing  streams  by 
which  they  were  excavated,  are  often  rilled  up  in  this 
way,  being  converted  into  irregular  veins  of  calcite. 
But  calcite  is  not  the  only  mineral  found  in  these 
cavern  deposits,  for  barite  and  fluorite,  and  various 
lead  and  zinc  ores,  especially  the  sulphides  of  these 
metals  —  galenite  and  sphalerite  —  have  also  been 
leached  out  of  the  surrounding  limestone  and  concen- 
trated in  the  caverns.  The  celebrated  lead  mines  of 
the  Mississippi  Valley,  and  some  of  the  richest  silver- 
lead  mines  of  Utah  and  Nevada  are  of  this  character, 


PETROLOGY.  165 

The  forms  of  these  cavern- deposits  vary  almost  indefi- 
nitely, and  are  often  highly  irregular.  The  principal 
types  are  known  as  gash-veins,  flats  and  sheets  (Fig. 
17),  chambers  and  pockets. 

Where  joints  and  other  cracks  have  opened  slightly 
in  different  directions  and  become  filled  with  infiltrated 
ores,  we  have  what  the  German  miners  call  a  stock- 
work,  —  an  irregular  network  of  small  and  interlacing 
veins. 

An  impregnation  is  an  irregular  segregation  of  met- 
alliferous minerals  in  the  mass  of  some  eruptive  or 


Fig.  17.  —  Gash-veins  and  sheets. 

crystalline  rock.     Its  outlines  are  not  sharply  defined, 
but  it  shades  off  gradually  into  the  enclosing  rock. 

Fahlbands  are  similar  ill-defined  deposits  or  segre- 
gations in  stratified  rocks.  An  impregnation  or  vein 
occurring  along  the  contact  between  two  dissimilar 
rocks  is  called  a  contact  deposit.  These  are  usually 
found  between  formations  of  different  geological  ages, 
and  especially  between  eruptive  and  sedimentary 
rocks. 


166  STRUCTURAL    GEOLOGY. 


Subsequent  Structures  produced  by  Subterranean 
Agencies. 

The  subterranean  forces  concerned  in  the  formation 
of  rocks  are  chiefly  various  manifestations  of  that 
enormous  tangential  pressure  developed  in  the  earth's 
crust,  partly  by  the  cooling  and  shrinkage  of  its 
interior,  but  largely,  it  is  probable,  by  the  diminution 
of  the  velocity  of  the  earth's  rotation  by  tidal  friction, 
and  the  consequent  diminution  of  the  oblateness  of  its 
form.  It  is  well  known  that  the  centrifugal  force  aris- 
ing from  the  earth's  rotation  is  sufficient  to  change  the 
otherwise  spherical  form  of  the  earth  to  an  oblate 
spheroid,  with  a  difference  of  twenty-six  miles  between 
the  equatorial  and  polar  diameters.  It  is  also  well 
known  that  while  the  earth  turns  from  west  to  east  on 
its  axis,  the  tidal  wave  moves  around  the  globe  from 
east  to  west,  thus  acting  like. a  powerful  friction-brake 
to  stop  the  earth's  rotation.  Our  day  is  consequently 
lengthening,  and  the  earth's  form  as  gradually  approach- 
ing the  perfect  sphere.  This  means  a  very  decided 
shortening  and  consequent  crumpling  of  the  equatorial 
circumference,  and  is  equivalent  to  a  marked  shrink- 
age of  the  earth's  interior,  so  far  as  the  equatorial 
regions  are  concerned. 

The  most  important  and  direct  result  of  the  hori- 
zontal thrust,  whether  due  to  cooling  or  tidal  friction, 
is  the  corrugation  or  wrinkling  of  the  crust ;  and  the 
earth-wrinkles  are  of  three  orders  of  magnitude,  — 
continents,  mountain-ranges,  and  rock- folds  or  arches. 

Continents  and  ocean-basins,  although  the  most 
important  and  permanent  structural  features  of  the 


PETROLOGY.  167 

earth's  crust,  do  not  demand  further  consideration 
nere,  since  their  forms  and  relations  are  adequately 
described  in  the  better  text-books  of  physical  geogra- 
phy. The  forms  and  distribution  of  mountain-ranges 
might  be  dismissed  in  the  same  way ;  but,  unlike  con- 
tinents, the  structure  of  mountains,  upon  which  their 
reliefs  mainly  depend,  is  quite  fully  exposed  to  our 
observation,  and  is  one  of  the  most  important  fields  of 
the  student  of  structural  geology.  Mountains,  how- 
ever, as  previously  explained,  combine  nearly  all  the 
kinds  of  structure  produced  by  the  subterranean  agen- 
cies, and  their  consideration,  therefore,  belongs  at  the 
end  rather  than  the  beginning  of  this  section. 

INCLINED  OR  FOLDED  STRATA.  —  Normally,  strata 
are  horizontal,  and  dikes  and  veins  are  vertical  or 
nearly  so.  Hence  the  stratified  rocks  are  more  ex- 
posed to  the  crumpling  action  of  the  tangential  pres- 
sure in  the  earth's  crust  than  the  eruptive  and  vein 
rocks  j  and  it  is  for  this  reason  and  partly  because  the 
stratified  rocks  are  vastly  more  abundant  than  the 
other  kinds,  that  the  effects  of  the  corrugation  of  the 
crust  are  studied  chiefly  in  the  former.  But  it  should 
be  understood  that  folded  dikes  and  veins  are  not 
uncommon. 

That  the  stratified  rocks  have,  in  many  instances, 
suffered  great  disturbance  subsequent  to  their  deposi- 
tion, is  very  evident ;  for,  while  the  strata  must  have 
been  originally  approximately  straight  and  horizontal, 
they  are  now  often  curved,  or  sharply  bent  and  con- 
torted, and  highly  inclined  or  even  vertical.  All  in- 
clined beds  or  strata  are  portions  of  great  folds  or 
arches.  Thus  we  may  feel  sure  when  we  see  a  stratum 


168  STRUCTURAL    GEOLOGY. 

sloping  downward  into  the  ground,  that  its  inclination 
or  dip  does  not  continue  at  the  same  angle,  but  that 
at  some  moderate  depth  it  gradually  changes  and  the 
bed  rises  to  the  surface  again.  Similarly,  if  we  look  in 
the  opposite  direction  and  think  of  the  bed  as  sloping 
upward  —  we  know  that  the  surface  of  the  ground  is  be- 
ing constantly  lowered  by  erosion,  and  consequently  that 
the  inclined  stratum  formerly  extended  higher  than  it 
does  now,  but  not  indefinitely  higher ;  for,  in  imagina- 
tion, we  see  it  curving  and  descending  to  the  level  of 
the  present  surface  again.  Hence  it  forms,  at  the 
same  time,  part  of  one  side  of  a  great  concave  arch, 
and  of  a  great  convex  arch,  just  as  every  inclined  sur- 
face on  the  ground  indicates  both  a  hill  and  a  valley. 
And  guided  by  this  principle  we  can  often  reconstruct 
with  much  probability  folds  that  have  been  more  or 
less  completely  swept  away  by  erosion,  or  that  are 
buried  beyond  our  sight  in  the  earth's  crust. 

The  arches  of  the  strata  are  rarely  distinctly  indi- 
cated in  the  topography,  but  must  be  studied  where 
the  ground  has  been  partly  dissected,  as  in  cliffs, 
gorges,  quarries,  etc.  They  are  also,  as  a  rule,  far 
more  irregular  and  complex  than  they  are  usually  con- 
ceived or  represented.  The  wrinkles  of  our  clothing 
are  often  better  illustrations  of  rock- folds  than  the 
models  and  diagrams  used  for  that  purpose.  This 
becomes  self-evident  when  we  reflect  that  the  earth's 
crust  is  exceedingly  heterogeneous  in  composition  and 
structure,  and  must,  therefore,  yield  unequally  to  the 
unequal  strains  imposed  upon  it. 

The  folds  or  undulations  of  the  strata  may  be  profit- 
ably compared  with  water-waves.  In  fact,  the  com- 


PETROLOGY. 


169 


parison  is  so  close  that  they  have  been  not  inaptly 
called  rock-waves.  Folds,  like  waves,-  unless  very 
large,  rarely  continue  for  any  great  distance,  but  die 
out  and  are  replaced  by  others,  giving  rise  to  the  en 
echelon  or  step-like  arrangement.  The  plan  of  both 
a  wave  and  a  fold  is  a  more  or  less  elongated  ellipse, 
each  stratum  in  a  fold  being  semi-ellipsoidal  or  boat- 
shaped.  In  other  words,  a  normal  fold  is  an  elongated 
mound  of  concentric  strata,  being  highest  at  the  cen- 


Fig.  18.  —  Anticlinal  and  synclinal  folds. 

tre,  sloping  very  gradually  toward  the  ends,  and  much 
more  abruptly  toward  the  sides. 

The  imaginary  line  passing  longitudinally  through  a 
fold,  about  which  the  strata  appear  to  be  bent,  is  the 
axis ;  and  the  plane  lying  midway  between  the  two 
sides  of  a  fold  and  including  the  axis  is  the  axial 
plane:  The  two  principal  kinds  of  folds  are  the  anti- 
cline (Fig.  1 8,  A),  where  the  strata  dip  away  from  the. 
axis;  and  the  syncline  (Fig.  18,  £},  where  they  dip 
toward  the  axis.  They  are  commonly,  but  not  al- 
ways, correlative,  like  hill  and  valley. 

Rock-folds  are  of  all  sizes,  from  almost  microscopic 


170  STRUCTURAL    GEOLOGY. 

wrinkles  to  great  arches  miles  in  length  and  breadth, 
and  thousands  of  feet  in  height.  The  smaller  folds, 
or  such  as  may  be  seen  in  hand  specimens  and  even 
in  considerable  blocks  of  stone,  are  commonly  called 
contortions,  and  it  is  interesting  to  observe  that  they 
are,  in  nearly  everything  except  size,  precisely  like  the 
large  folds,  so  that  they  answer  admirably  as  geological 
models.  Large  folds,  however,  are  almost  necessarily 
curves,  but  contortions  are  frequently  angular  (Fig. 
19).  With  folds,  as  with  waves,  the  small  undulations 
are  borne  upon  the  large  ones ;  but  the  contortions 
are  not  uniformly  distributed.  An  inspection  of  Fig. 


Fig.  19.  —  Contorted  strata. 

1 8  shows  that  when  the  rocks  are  folded  they  must  be 
in  a  state  of  tension  on  the  anticlines  (A],  and  in  a 
state  of  compression  in  the  synclines  (B),  and  the 
latter  is  evidently  the  normal  position  of  the  pucker- 
ings  or  contortions  of  the  strata,  as  shown  in  Fig.  20. 
Contortions  are  also  most  commonly  found  in  thin- 
bedded,  flexible  rocks,  such  as  shales  and  schists. 
And  when  we  find  them  in  hard,  rigid  rocks,  like 
gneiss  and  limestone,  it  must  mean  either  that  the 
structure  was  developed  with  extreme  slowness,  or 
that  the  rock  was  more  flexible  then  and  possibly 
plastic. 

It  is  very  interesting  to  notice  the  relations  of  anti- 


PETROLOGY. 


171 


clinal  and  synclinal  folds  to  the  agents  of  erosion.  At 
the  time  the  folds  are  made,  the  anticlinals,  of  course, 
are  ridges,  and  the  synclinals,  valleys,  and  this  relation 


Fig.  20.  —  Contorted  syncline. 

sometimes  continues,  as  shown  in  Fig.  21 ;  but  we 
have  seen  that  the  rocks  in  the  trough  of  the  synclinal 
are  compressed  and  compacted,  i.e.,  made  more  capa- 


Fig.  21.  —  Section  of  anticlinal  mountains. 

ble  of  resisting  erosion,  while  those  on  the  crest  of  the 
anticlinal  are  stretched  and  broken,  i.e.,  made  more 
susceptible  of  erosion.  The  consequence  is  that  the 
anticlinals  are  usually  worn  away  very  much  faster 


172  STRUCTURAL   GEOLOGY. 

than  the  synclinals ;  so  much  faster  that  in  many  cases 
the  topographic  features  are  completely  transposed, 
and  in  place  of  anticlinal  ridges  and  synclinal  valleys 


Fig.  22.  —  Section  of  synclinal  mountains. 

(Fig.  21)  we  find  synclinal  ridges  and  anticlinal  val- 
leys (Fig.  22). 

Besides   the  anticlinal  and  synclinal   folds  already 
explained,  there  are  folds  that  slope  in  only  one  direc* 


Fig.  23.  —  Monoclinal  fold. 

tion,  one-sided  or  monoclinal  folds  (Fig.  23).  Anti- 
clinal and  synclinal  folds  are  symmetrical 'when  the  dip 
or  slope  of  the  strata  is  the  same  on  both  sides  and 
the  axial  plane  is  vertical.  The  great  majority  of 


PETROLOGY.  173 

folds,  however,  are  unsymmetrical,  the  opposite  slopes 
being  unequal,  and  the  axial  planes  inclined  to  the 
vertical  (Fig.  24,  A).  This  means  that  the  compress- 
ing or  plicating  force  has  been  greater  from  one  side 
than  from  the  other,  as  indicated  by  the  arrows.  It 
acted  with  the  greatest  intensity  on  the  side  of  the 
gentler  slope,  the  tendency  evidently  having  been  to 
crowd  or  tip  the  fold  over  in  the  direction  of  the  steep 
slope.  When  the  steep  slope  approaches  the  vertical, 
this  tendency  is  almost  unresisted,  and  when  it  passes 
,the  vertical,  gravitation  assists  in  overturning  the  fold 
(Fig.  24,  B).  Such  highly  unsymmetrical  folds,  in- 


Fig.  24.  —  Unsymmetrical  and  inverted  folds. 

eluding  all  cases  where  the  two  sides  of  the  fold  slope 
in  the  same  direction,  are  described  as  overturned  or 
inverted,  although  the  latter  term  is  not  strictly  appli- 
cable to  the  entire  fold,  but  only  to  the 'strata  compos- 
ing the  under  or  lee  side  of  it.  Fig.  24,  B,  shows  that 
these  beds  are  completely  inverted,  the  older,  as  the 
figures  indicate,  lying  conformably  upon  the  newer. 
This  inversion  is  one  of  the  most  important  features 
of  folded  strata,  and  it  has  led  «  to  many  mistakes  in 
determining  their  order  of  succession.  In  the  great 
mountain-chains,  especially,  it  is  exhibited  on  the 
grandest  scale,  great  groups  of  strata  being  folded 
over  and  over  each  other  as  we  might  fold  carpets. 


174 


STRUCTURAL    GEOLOGY. 


An  inverted  stratum  is  like  a  flattened  S  or  Z,  and  may 
be  pierced  by  a  vertical  shaft  three  times,  as  has 
actually  happened  in  some  coal  mines.  Folds  are 
open  when  the  sides  are  not  parallel,  and  closed  when 
they  are  parallel,  the  former  being  represented  by  a 
half-open,  and  the  latter  by  a  closed,  book.  Closed 
folds  are  usually  inverted,  and  when  the  tops  have 
been  removed  by  erosion  (Fig.  25),  the  repetition  of 
the  strata  may  escape  detection,  and  the  thickness  of 
the  section  be,  in  consequence,  greatly  overestimated. 


Fig.  25.  —  Series  of  closed  folds. 

Thus,  a  geologist  traversing  the  section  in  Fig.  25 
would  see  thirty-two  strata,  all  inclined  to  the  left  at 
the  same  angle,  those  on  the  right  apparently  passing 
below  those  on  the  left,  and  all  forming  part  of  one 
great  fold.  The  repetition  of  the  strata  in  reverse 
order,  as  indicated  by  the  numbers,  and  the  structure 
below  the  surface,  show,  however,  that  the  section 
really  consists  of  only  four  beds  involved  in  a  series  of 
four  closed  folds,  the  true  thickness  of  the  beds  in  this 
section  being  only  one-eighth  as  great  as  the  apparent 
thickness. 

The  most  important  features  to  be  noted  in  observ- 
ing and  describing  inclined  or  folded  strata  are  the 
strike  and  dip.  The  strike  is  the  compass  bearing  or 


PETROLOGY.  175 

horizontal  direction  of  the  strata.  It  is  the  direction 
of  the  outcrop  of  the  strata  where  the  ground  is  level. 
It  may  also  be  denned  as  the  direction  of  a  level  line 
on  the  surface  of  a  stratum,  and  is  usually  parallel  with 
the  axis  of  the  fold. 

The  dip  is  the  inclination  of  the  beds  to  the  plane 
of  the  horizon,  and  embraces  two  elements  :  (a)  the 
direction  of  the  dip,  which  is  always  at  right  angles  to 
the  strike,  being  the  line  of  steepest  descent  on  the 
surface  of  the  stratum,  and  (b}  the  amount  of  the 


Fig.  26.—  Dip  and  strike. 

dip,  which  is  the  value  of  the  angle  between  the  line 
of  steepest  descent  and  the  horizon. 

In  Fig.  26,  s  t  is  the  direction  of  the  strike,  and  dp 
that  of  the  dip.  The  strike  and  direction  of  the  dip 
are  determined  with  the  compass,  and  the  amount  of 
the  dip  with  the  clinometer,  an  instrument  for  measur- 
ing vertical  angles. 

The  strike  is  much  less  variable  than  the  dip,  being 
often  essentially  constant  over  extensive  districts ; 
while  the  dip,  except  in  very  large  or  closed  folds,  is 
constantly  changing  in  direction  and  amount. 

When  the  dip  and  surface  breadth  of  a  series  of 
strata  have  been  measured,  it  is  a  simple  problem  in 


I76  STRUCTURAL    GEOLOGY. 

trigonometry  to  determine  the  true  thickness,  and  the 
depth  below  the  surface  of  any  particular  stratum  at 
any  given  distance  from  its  outcrop.  When  the  strata 
are  vertical,  the  surface  breadth  or  traverse  measure 
is  equal  to  the  thickness.  ' 

By  the  outcrop  of  a  stratum  or  formation  we  ordi- 
narily understand  its  actual  exposure  on  the  surface, 
where  it  projects  through  the  soil  in  ledges  or  quarries. 
But  the  term  is  also  more  broadly  defined  to  mean  the 
exposure  of  the  stratum  as  it  would  appear  if  the  soil 
were  entirely  removed.  It  is  instructive  to  observe 
the  relations  of  the  outcrop  to  the  form  of  the  surface. 
Its  breadth  varies  with  its  inclination  to  the  surface, 
appearing  narrow  and  showing  its  true  thickness  where 
it  is  perpendicular  to  the  surface,  and  broadening  out 
rapidly  where  the  surface  cuts  it  obliquely.  The  out- 
crops of  horizontal  strata  form  level  lines  or  bands 
along  the  sides  of  hills  and  valleys,  essentially  contour 
lines  in  the  topography ;  and  appear  as  irregular,  sinu- 
ous bands  bordering  the  streams  and  valleys  in  the 
map- view  of  the  country.  The  outcrops  of  vertical 
strata,  dikes,  or  veins,  on  the  other  hand,  are  repre- 
sented by  straight  lines  and  bands  on  the  map.  While 
the  outcrops  of  inclined  strata  are  deflected  to  the 
right  or  left  in  crossing  ridges  and  valleys,  according 
to  the  direction  and  amount  of  their  inclination. 

A  geological  map  shows  the  surface  distribution  of 
the  rocks,  t.e.,  gives  in  one  view  the  forms  and  arrange- 
ment of  the  outcrops  of  all  the  rocks  in  the  district 
mapped,  including  the  trend  or  strike  of  the  folded 
strata.  The  map  may  be  lithological,  each  kind  of 
rock,  as  granite,  sandstone,  limestone,  etc.,  being  rep- 


PETROLOGY.  177 

resented  by  a  different  color ;  or,  it  may  be  historical, 
each  color  representing  one  geological  formation,  /.<?., 
the  rocks  formed  during  one  period  of  geological  time, 
without  reference  to  their  lithological  character.  But 
in  the  best  maps  these  two  methods  are  combined. 
The  geological  section  shows  the  arrangement  of  the 
rocks  below  the  surface,  revealing  the  dip  of  the  strata 
and  supplementing  the  map,  both  modes  of  represen 
tation,  the  horizontal  and  vertical,  being  required  to 
give  a  complete  idea  of  the  geological  structure  of  a 
country.  For  a  detailed  and  satisfactory  explanation 
of  the  construction  and  use  of  geological  maps  and 
sections,  students  are  referred  to  Prof.  Geikie's  "  Out- 
lines of  Field  Geology." 

CLEAVAGE  STRUCTURE. — This  important  structure 
is  now  known  to  be,  like  rock-folds,  a  direct  result  of 
the  great  horizontal  pressure  in  the  earth's  crust.  It 
is  entirely  distinct  in  its  nature  and  origin  from  crys- 
talline cleavage,  and  may  properly  be  called  lithologic 
cleavage.  It  is  also  essentially  unlike  stratification  and 
joint-structure.  It  agrees  with  stratification  in  dividing 
the  rocks  into  thin  parallel  layers,  but  the  cleavage 
planes  are  normally  vertical  instead  of  horizontal.  And 
the  cleavage  planes  differ  from  joints  in  running  in 
only  one  direction,  dividing  the  rock  into  layers  j 
while  joints,  as  we  shall  see,  traverse  the  same  mass  of 
rock  in  various  directions,  dividing  it  into  blocks. 

The  principal  characteristics  of  lithologic  cleavage 
are :  (i)  It  is  rare,  except  in  fine-grained,  soft  rocks, 
having  its  best  development  in  the  slates,  roofing  slates 
and  school  slates  affording  typical  examples.  Hence 
it  is  commonly  known  as  slaty  cleavage.  (2)  The 


78 


STRUCTURAL    GEOLOGY. 


cleavage  planes  are  highly  inclined  or  vertical,  very 
constant  in  dip  and  strike,  and  quite  independent  of 
stratification.  (3)  It  is  usually  associated  with  folded 
strata,  and  often  with  distorted  nodules  or  fossils.  The 
more  important  of  these  characteristics  are  illustrated  by 

Fig.  27.  This 
represents  a 
block  cf  con- 
torted strata 
in  which  the 
dark  layers 
are  slate  with 
very  perfect 
cleavage  par- 
allel to  the 
left  -  hand 
shaded  side 
of  the  block ; 
while  the 
white  layers 
are  sandstone 
and  quite 
destitute  of 
cle  avage. 
Many  expla- 
nations of 

this  interesting  structure  have  been  proposed,  but  that 
first  advanced  by  Sharpe  may  be  regarded  as  fully 
established.  He  said  that  slaty  cleavage  is  always 
due  to  powerful  pressure  at  right  angles  to  the  planes 
of  cleavage.  All  the  characteristics  of  cleavage  noted 
above  are  in  harmony  with  this  theory.  Cleavage  is 


Fig.  97.  —  Slaty  cleavage  in  contorted  strata. 


PETROLOGY.  179 

limited  to  fine-grained  or  soft  rocks,  because  these 
alone  can  be  modified  internally  by  pressure,  without 
rupture.  Harder  and  more  rigid  rocks  may  be  bent 
or  broken,  but  they  appear  insusceptible  of  minute 
wrinkling  or  other  change  of  structure  affecting  every 
particle  of  the  mass.  Since  the  cleavage  planes  are 
normally  vertical,  the  pressure,  according  to  the  theory, 
must  be  horizontal.  That  this  horizontal  pressure 
exists  and  is  adequate  in  direction  and  amount,  is 
proved  by  the  folds  and  contortions  of  the  cleaved 
strata;  for,  as  shown  in  Fig.  27,  the  cleavage  planes 
coincide  with  the  strike  of  the  foldings,  and  are  thus 
perpendicular  to  the  pressure  horizontally  as  well  as 
vertically.  The  distortion  of  the  fossils  in  cleaved 
slates  is  plainly  due  to  pressure  at  right  angles  to  the 
cleavage,  for  they  are  compressed  or  shortened  in  that 
direction,  and  extended  or  flattened  out  in  the  planes 
of  cleavage.  Again,  Tyndall  has  shown  that  the  mag- 
netism of  cleaved  slate  proves  that  it  has  been  power- 
fully compressed  perpendicularly  to  the  cleavage.  And, 
finally,  repeated  experiments  by  Sorby  and  others  have 
proved  that  a  very  perfect  cleavage  may  be  developed 
in  clay  (unconsolidated  slate)  by  compression,  the 
planes  of  cleavage  being  at  right  angles  to  the  line  of 
pressure.  When,  however,  Sharpe's  theory  had  been 
thus  fully  demonstrated,  the  question  as  to  how  pres- 
sure produces  cleavage  still  remained  unanswered. 
Sorby  held  that  clay  contains  foreign  particles  with 
unequal  axes,  such  as  mica-scales,  etc.,  and  that  these 
are  turned  by  the  pressure  so  as  to  lie  in  parallel 
planes  perpendicular  to  its  line  of  action,  thus  produc- 
ing easy  splitting  or  cleavage  in  those  planes.  And 


l8o  STRUCTURAL    GEOLOGY. 

he  proved  by  experiments  that  a  mixture  of  clay  and 
mica-scales  does  behave  in  this  way.  But  Tyndall 
showed  that  the  cleavage  is  more  perfect  just  in  pro- 
portion as  the  clay  is  free  from  foreign  particles,  and 
in  such  a  perfectly  homogeneous  substance  as  beeswax, 
he  developed  a  more  perfect  cleavage  than  is  possible 
in  clay.  His  theory,  which  is  now  universally  accepted, 
is,  that  the  clay  itself  is  composed  of  grains  which  are 
flattened  by  pressure,  the  granular  structure  with 
irregular  fracture  in  all  directions,  changing  to  a  scaly 
structure  with  very  easy  and  plane  fracture  or  splitting 
in  one  definite  direction. 

Observations  on  distorted  fossils  and  nodules  have 
shown  that  when  slaty  cleavage  is  developed,  the  rock 
is,  on  the  average,  reduced  in  the  direction  of  the 
pressure  to  two-fifths  of  its  original  extent,  and  corre- 
spondingly extended  in  the  vertical  direction.  Thus, 
whether  rocks  yield  to  the  horizontal  pressure  in  the 
earth's  crust,  by  folding  and  corrugation,  or  by  the 
flattening  of  their  constituent  particles,  they  are  alike 
shortened  horizontally  and  extended  vertically ;  and  it 
is  impossible  to  overestimate  the  importance  of  these 
facts  in  the  formation  of  mountains. 

FAULTS  OR  DISPLACEMENTS.  —  We  may  readily  con- 
ceive that  the  forces  which  were  adequate  to  elevate, 
corrugate,  and  even  crush  vast  masses  of  solid  rock 
were  also  sufficient  to  crack  and  break  them ;  and 
eince  the  fractures  indicate  that  the  strains  have  been 
applied  unequally,  it  will  be  seen  that  unequal  move- 
ments of  the  parts  must  often  result.  If  this  unequal 
movement  takes  place,  i.e.,  if  the  rocks  on  opposite 
sides  of  a  fracture  of  the  earth's  crust  do  not  move 


PETROLOGY.  l8l 

together,  but  slip  over  each  other,  a  fault  is  produced. 
The  two  sides  may  move  in  opposite  directions,  or  in 
the  same  direction  but  unequally,  or  one  side  may 
remain  stationary  while  the  other  moves  up  or  down. 
It  is  simply  essential  that  the  movement  should  be 
unequal  in  direction,  or  amount,  or  both ;  that  there 
should  be  an  actual  slip,  so  that  strata  that  were  once 
continuous  no  longer  correspond  in  position,  but  lie 
at  different  levels  on  opposite  sides  of  the  fracture. 
The  vertical  difference  in  movement  is  known  as  the 
throw,  slip,  or  displacement  of  the  fault.  Fault-fract- 


Fig.  28.  —  Section  of  a  normal  fault.       Fig.  29.  —  Section  of  a  reversed  fault. 

ures  rarely  approach  the  horizontal  direction,  but  are 
usually  highly  inclined  or  approximately  vertical. 
When  the  fault  is  inclined,  as  in  Fig.  28,  the  actual 
slipping  in  the  plane  of  the  fault  exceeds  the  vertical 
throw,  for  the  movement  is  then  partly  horizontal,  the 
beds  being  pulled  apart  endwise.  The  inclination  of 
faults,  as  of  veins  and  dikes,  should  be  measured  from 
the  vertical  and  called  the  hade.  Faults  are  some- 
times hundreds  of  miles  in  length ;  and  the  throw 
may  vary  from  a  fraction  of  an  inch  to  thousands  of 
feet. 

Transverse   sections,    such   as   are   represented   by 
Fig.  28  and  many  specimens  and  models,  do  not  give 


l82  STRUCTURAL    GEOLOGY. 

the  complete  plan  or  idea  of  a  fault ;  but  this  is  seen 
more  perfectly  in  Fig.  30.  We  learn  from  this  that  a 
typical  fault  is  a  fracture  along  which  the  strata  have 
sagged  or  settled  down  unequally.  The  most  impor- 
tant point  to  be  observed  here  is  that  the  strata  do 
not  drop  bodily,  but  are  merely  bent,  the  throw  being 
greatest  at  the  middle  of  the  fault  and  gradually  dimin- 
ishing toward  the  ends.  In  other  words,  every  simple 
fault  must  die  out  gradually ;  for  we  cannot  conceive 
of  a  fault  as  ending  abruptly,  except  where  it  turns 


Fig.  30.  —  Ideal  view  of  a  complete  fault. 

upon  itself  so  as  to  completely  enclose  a  block  of  the 
strata,  which  may  drop  down  bodily ;  but  the  fault  is 
then  really  endless.  A  fault  may  be  represented  on 
a  map  by  a  line ;  if  a  simple  fault,  by  a  single  straight 
line.  But  faults  are  often  compound,  and  are  repre- 
sented by  branching  lines;  that  is,  the  earth's  crust 
has  been  broken  irregularly,  and  the  parts  adjoining 
the  fracture  have  sagged  or  risen  unequally. 

The  rock  above  an  inclined  fault,  vein,  or  dike 
(Fig.  28)  is  called  the  hanging  wall,  and  that  below 
the  foot  wall.  Now  inclined  faults  are  divided  into 
two  classes,  according  to  the  relative  movements  of 


PETROLOGY.  183 

the  two  walls.  Usually,  the  hanging  wall  slips  down 
and  the  foot  wall  slips  up,  as  in  Fig.  28.  Faults  on 
this  plan  are  so  nearly  the  universal  rule  that  they  are 
called  normal  faults.  They  indicate  that  the  strata 
were  in  a  state  of  tension,  for  their  broken  ends  are 
pulled  apart  horizontally,  so  that  a  vertical  line  may 
cross  the  plane  of  a  stratum  without  touching  it. 

A  few  important  faults  have  been  observed,  however, 
in  which  the  foot-wall  has  fallen  and  the  hanging-wall 
has  risen  (Fig.  29).  These  are  known  as  reversed 


Fig.  31.  —  Explanation  of  normal  faults. 

faults ;  and  they  indicate  that  the  strata  were  in  a  state 
of  lateral  compression,  the  broken  ends  of  the  beds 
having  been  pushed  horizontally  past  each  other,  so 
that  a  vertical  line  or  shaft  may  intersect  the  same  bed 
twice,  as  has  been  actually  demonstrated  in  the  case 
of  some  beds  of  coal. 

The  usual  explanation  of  normal  faults  is  given  in 
Fig.  31.  The  inclined  fractures  of  the  earth's  crust 
must  often  be  converging,  bounding,  or  enclosing  large 
V-shaped  blocks  (A,  J5) .  If  now,  through  any  cause, 
as  the  folding  of  the  strata,  they  are  brought  into  a 


184  STRUCTURAL    GEOLOGY. 

state  of  tension,  so  that  the  fractures  are  widened,  the 
V-shaped  masses,  being  unsupported,  settle  down,  the 
fractures  bounding  them  becoming  normal  faults,  as 
is  seen  by  tracing  the  bed  X  through  the  dislocations. 
The  single  fracture  below  the  block  A  is  inclined,  and 
the  stretching  has  been  accomplished  by  slipping  along 
it  and  faulting  the  bed  Z  as  well  as  X,  the  entire  section 
to  the  right  of  this  fracture  being  part  of  a  much  larger 
V-shaped  block  the  right-hand  boundary  of  which  is 
not  seen.  But  the  united  fracture  below  the  block  B 
being  vertical,  any  horizontal  movement  must  widen  it 
into  a  fissure,  which  is  kept  open  by  the  great  wedge 
above  and  may  become  the  seat  of  a  dike  or  mineral 
vein.  The  beds  below  the  V  may,  in  this  case,  escape 
dislocation,  as  is  seen  by  tracing  the  bed  Z  across  the 
fissure.  These  pairs  of  converging  normal  faults  are 
called  trough  faults ;  and  this  is  the  only  way  in  which 
we  can  conceive  of  important  faults  as  terminating  at 
moderate  depths  below  the  surface,  and  not  affecting 
the  entire  thickness  of  the  earth's  crust. 

Important  reversed  faults  are  believed  to  occur 
chiefly  along  the  axes  of  overturned  anticlines  (Fig. 
24)  where  the  strata  have  been  broken  by  the  unequal 
strains,  and  those  on  the  upper  side  shoved  bodily 
over  those  on  the  lower  or  inverted  side. 

An  extensive  displacement  of  the  strata  is  sometimes 
accomplished  by  short  slips  along  each  of  a  series  of 
parallel  fractures,  producing  a  step  fault. 

Faults  cutting  inclined  or  folded  strata  are  divided 
into  two  classes,  according  as  they  are  approximately 
parallel  with  the  direction  of  the  dip  or  of  the  strike. 
The  first  are  known  as  transverse  or  dip  faults,  and 


PETROLOGY.  185 

the  second  as  longitudinal  or  strike  faults.  The  chief 
interest  of  either  class  consists  in  their  effect  upon  the 
outcrops  of  the  faulted  strata,  after  erosion  has  removed 
the  escarpment  produced  by  the  dislocation. 

Dip  faults  cause  a  lateral  shift  or  displacement  of 
the  outcrops,  as  shown  in  Fig.  32,  which  represents  a 
plan  or  map-view  of  the  strata  traversed  by  the  fault 
b  b,  the  down  throw  being  on  the  right  and  the  up 
throw  on  the  left.  The  dip  of  the  strata  is  indicated 
by  the  small  arrows  and  the  accompanying  figures ; 


Fig.  32.  —  Plan  of  a  dip  fault. 

and  it  will  be  observed  on  tracing  the  outcrop  of  any 
stratum,  a  a,  across  the  fault  that  it  is  shifted  to  the 
right.  If  the  throw  of  the  fault  were  reversed,  the 
displacement  of  the  outcrop  would  be  reversed  also. 
Strike  faults  are  of  two  kinds,  according  as  they  incline 
in  the  same  direction  as  the  strata,  or  in  the  contrary 
direction.  The  effect  of  the  first  kind  is  to  conceal 
some  of  the  beds,  as  shown  in  Fig.  33,  in  which 
beds  5  and  6  do  not  outcrop,  but  we  pass  on  the  sur- 
face abruptly  from  4  to  7.  The  apparent  thickness  of 
the  section  is  thus  less  than  the  real  thickness.  When 
the  fault  inclines  against  the  strata,  on  the  other  hand 


186  STRUCTURAL    GEOLOGY. 

(Fig.  34),  the  outcrops  of  certain  strata  are  repeated 
on  the  surface  ;  and  a  number  of  parallel  faults  of  this 
kind,  a  step  fault,  will,  like  a  series  of  closed  folds 
(Fig.  25),  cause  the  apparent  thickness  of  the  section 
to  greatly  exceed  the  real  thickness.  Repetition  of  the 
strata  by  faulting  is  distinguished  from  repetition  by 
folding  by  being  in  the  same  instead  of  the  reverse 
order. 

Folds  and  faults  are  really  closely  related.  In  the 
former  the  strata  are  disturbed  and  displaced  by  bend- 
ing ;  in  the  latter  by  breaking  and  slipping ;  and  the 

4 G  9      _*_     a       B 4 


Fig.  33.  —  Strike  fault,  concealing  Fig.  34.  —  Strike  fault,  repeating 

strata.  strata. 


displacement  which  is  accomplished  by  a  fold  may 
gradually  change  to  a  fracture  and  slip.  This  relation 
is  especially  noticeable  with  monoclinal  folds  (Fig.  23), 
in  which  the  tendency  to  shear  or  break  the  beds  is 
oftei)  very  marked. 

Important  faults  are  rarely  simple,  well-defined  frac- 
tures ;  but,  in  consequence  of  the  enormous  friction, 
the  rocks  are  usually  more  or  less  broken  and  crushed, 
sometimes  for  a  breadth  of  many  feet  or  yards.  The 
fragments  of  the  various  beds  are  then  strung  along 
the  fault  in  the  direction  of  the  slipping,  and  this  cir- 
cumstance has  been  made  use  of  in  tracing  the  con- 
tinuation of  faulted  beds  of  coal.  In  other  cases  the 


PETROLOGY.  187 

direction  of  the  slip  is  plainly  indicated  by  the  bend- 
ing of  the  broken  ends  of  the  strata  (Fig.  35),  and  the 
beds  are  sometimes  turned  up  at  a  high  angle  or  even 
overturned  in  this  way. 

Since  faults  are  not  plane,  but  undulating  and  often 
highly  irregular,  fractures,  the  walls  will  not  coincide 
after  slipping;  and  if  the  rocks  are  hard  enough  to 
resist  the  enormous  pressure,  the  cavities  or  fissures 
produced  in  this  way  may  remain  open.  Now  faults 
are  continuous  fractures  of  the  earth's  crust,  reaching 


Fig-  35«  —  Section  of  beds  distorted  by  a  fault. 

down  to  an  unknown  but  very  great  depth ;  and  hence 
they  afford  the  best  outlets  for  the  heated  subterranean 
waters ;  so  that  it  is  common  to  find  an  important 
fault  marked  on  the  surface  by  a  line  of  springs,  and 
these  are  often  thermal.  The  warm  mineral  waters  on 
their  way  to  the  surface  deposit  part  of  the  dissolved 
minerals  in  the  irregular  fissures  along  the  fault,  which 
are  thus  changed  to  mineral  veins.  This  agrees  with 
the  fact  that  the  walls  of  veins  usually  show  faulting 
as  well  as  crushed  rock,  slickensides,  and  other  evi- 
dences of  slipping. 

If  the  earth's  surface  were  not  subject  to  erosion, 


i88  STRUCTURAL  GEOLOGY. 

every  fault  would  be  marked  on  the  surface  by  an 
escarpment  equal  in  height  to  the  throw  of  the  fault ; 
and,  notwithstanding  the  powerful  tendency  of  erosion 
to  obliterate  them,  these  escarpments  are  sometimes 
observed,  although  of  diminished  height.  Thus,  ac- 
cording to  Gilbert,  the  Zandia  Mountains  in  New 
Mexico  are  due  to  a  fault  of  1 1,000  feet,  leaving  an 
escarpment  still  7,000  feet  high.  But,  as  a  rule,  there 
is  no  escarpment  or  marked  inequality  of  the  surface, 
the  fault,  like  the  fold,  not  being  distinctly  indicated 
in  the  topography.  In  all  such  cases  we  must  con- 
clude either  that  the  faults  were  made  a  very  long 
time  ago,  or  that  they  have  been  formed  with  extreme 
slowness,  so  slowly  that  erosion  has  kept  pace  with  the 
displacement,  the  escarpments  being  worn  away  as 
fast  as  formed.  These  and  other  considerations  make 
it  quite  certain  that  extensive  displacements  are  not 
produced  suddenly,  but  either  grow  by  a  slow,  creep- 
ing motion,  or  by  small  slips  many  times  repeated  at 
long  intervals  of  time. 

JOINTS  AND  JOINT-STRUCTURE. — This  is  the  most 
universal  of  all  rock-structures,  since  all  hard  rocks 
and  many  imperfectly  consolidated  kinds,  like  clay, 
are  jointed.  Joints  are  cracks  or  planes  of  division 
which  are  usually  approximately  vertical  and  traverse 
the  same  mass  of  rock  in  several  different  directions. 
They  are  distinguished  from  stratification  planes  by 
being  rarely  horizontal,  and  from  both  stratification 
and  cleavage  planes  by  being  actual  cracks  or  frac- 
tures, and  by  dividing  the  rock  into  blocks  instead  of 
sheets  or  layers.  The  art  of  quarrying  consists  in 
removing  these  natural  blocks  ;  and  most  of  the  broad. 


PETROLOGY.  189 

flat  surfaces  of  rock  exposed  in  quarries,  are  the  joint- 
planes  (Fig.  36).  Some  of  the  most  familiar  features 
of  rock-scenery  are  also  due  to  this  structure,  cliffs, 
ravines,  etc.,  being  largely  determined  in  form  and 
direction  by  the  principal  systems  of  joints ;  and  we 
have  already  seen  that  the  same  is  true  of  veins  and 
dikes. 

Joints  are  divided  by  their  characteristics  and  modes 
>f  origin  into  three  classes  as  follows  :  — 


Fig.  36.  —  Quarry  showing  two  systems  of  parallel  joints. 

i .  The  parallel  and  intersecting  joints.  —  This  is  by 
far  the  most  important  class,  and  has  its  best  develop- 
ment in  stratified  rocks,  such  as  sandstone,  slate,  lime- 
stone, etc.  These  joints  are  straight  and  continuous 
cracks  which  may  often  be  traced  for  considerable 
distances  on  the  surface.  They  usually  run  in  several 
definite  directions,  being  arranged  in  sets  or  systems 
by  their  parallelism.  Thus  in  Fig.  36  one  set  of 
joints  is  represented  by  the  broad,  flat  surfaces  in 


190  STRUCTURAL   GEOLOGY. 

light,  and  a  second  set  crossing  the  first  nearly  at  right 
angles,  by  the  narrower  faces  in  shadow.  By  the  inter- 
sections of  the  different  sets  of  joints  the  rock  is 
divided  into  angular  blocks. 

Although  many  explanations  of  this  class  of  joints 
have  been  proposed,  it  has  long  been  the  general 
opinion  of  geologists  that  they  are  due  to  the  contrac- 
tion of  the  rocks,  i.e.,  that  they  are  shrinkage  cracks. 
We  shall  soon  see,  however,  that  they  lack  the  most 
important  characters  of  cracks  known  to  be  due  to 
shrinkage ;  and  the  present  writer  has  advanced  the 
view  that  movements  of  the  earth's  crust,  and  especially 
the  swift,  vibratory  movements  known  as  earthquakes, 
are  a  far  more  adequate  and  probable  cause.  It  is 
well  known  that  earthquakes  break  the  rocks  ;  and,  if 
space  permitted,  it  could  be  shown  that  the  earthquake- 
fractures  must  possess  all  the  essential  features  of  par- 
allel and  intersecting  joints. 

2.  Contraction  joints  or  shrinkage  cracks.  —  That 
many  cracks  in  rocks  are  due  to  shrinkage,  there  can 
be  no  doubt.  The  shrinkage  may  result  from  the 
drying  of  sedimentary  rocks ;  but  more  generally  from 
the  cooling  of  eruptive  rocks.  Every  one  has  noticed 
in  warm  weather,  the  cracks  in  layers  of  mud  or  clay 
on  the  shore,  or  where  pools  of  water  have  dried  up ; 
and  we  have  already  seen  that  these  sun-cracks  are 
often  preserved  in  the  hard  rocks.  They  have  certain 
characteristic  features  by  which  they  may  be  distin- 
guished from  the  joints  of  the  first  class.  They  divide 
the  clay  into  irregular,  angular  blocks,  which  often 
show  a  tendency  to  be  hexagonal  instead  of  quad- 
rangular. The  cracks  are  continually  uniting  and 


PETROLOGY.  191 

dividing,  but  are  not  parallel,  and  rarely  cross  each 
other.  Sun-cracks  never  affect  more  than  a  few  feet 
in  thickness  of  clay,  and  are  an  insignificant  structural 
feature  of  sedimentary  rocks.  In  eruptive  rocks,  on  the 
other  hand,  the  contraction  joints  have  a  very  exten- 


Fig.  37.  —  Columnar  dike. 

sive,  and,  in  some  cases,  a  very  perfect  development, 
culminating  in  the  prismatic  or  columnar  jointing  of 
the  basaltic  rocks.  This  remarkable  structure  has 
long  excited  the  interest  of  geologists,  and,  although 
the  basalt  columns  were  once  regarded  as  crystals,  and 
later  as  a  species  of  concretionary  structure,  it  is  now 
generally  recognized  as  the  normal  result  of  slow  cool- 


192  STRUCTURAL    GEOLOGY. 

ing  in  a  homogeneous,  brittle  mass.  The  columns  are 
normally  hexagonal,  and  perpendicular  to  the  cooling 
surface,  being  vertical  in  horizontal  sheets  and  lava 
flows,  as  in  the  classic  examples  of  the  Giant's  Cause- 
way and  Fingal's  Cave,  and  horizontal  in  vertical 
dikes  (Fig.  37).  They  begin  to  grow  on  the  cooling 
surface  of  the  mass  and  gradually  extend  toward  the 
centre,  so  that  dikes  frequently  show  two  independent 
sets  of  columns. 

3.  The  concentric  joints  of  granitic  rocks.  —  In 
quarries  of  granite  and  other  massive  crystalline  rocks, 
it  is  often  very  noticeable  that  the  rock  is  divided  into 
more  or  less  regular  layers  by  cracks  which  are  ap- 
proximately parallel  with  the  surface  of  the  ground, 
some  of  the  granite  hills  having  thus  a  structure  re- 
sembling that  of  an  onion.  The  layers  are  thin  near 
the  surface,  become  thicker  and  less  distinct  down- 
wards, and  cannot  usually  be  traced  below  a  depth  of 
fifty  or  sixty  feet.  These  concentric  cracks  are  of 
great  assistance  in  quarrying,  and  are  now  regarded  as 
due  to  the  expansion  of  the  superficial  portions  of  the 
granite  caused  by  the  heat  of  the  sun.  In  reference 
to  this  view  of  their  origin  these  may  be  properly 
called  expansion  joints. 

STRUCTURE  OF  MOUNTAIN-CHAINS.  —  Mountains  are 
primarily  of  two  kinds,  —  volcanic  and  non- volcanic. 
The  structure  of  the  former  belongs  properly  with  the 
original  structures  of  the  volcanic  rocks ;  but  the 
latter  —  the  true  mountains  —  owe  their  internal  struc- 
ture and  altitude  or  relief  almost  wholly  to  the  crump- 
ling and  mashing  together  of  great  zones  of  the  earth's 
crust,  being,  as  already  pointed  out,  the  culminating 


PETROLOGY.  193 

points  of  the  plication,  cleavage,  and  faulting  of  the 
strata.  "  A  mountain-<:to'#  consists  of  a  great  plateau 
or  bulge  of  the  earth's  surface,  often  hundreds  of  miles 
wide  and  thousands  of  miles  long.  This  is  usually 
more  or  less  distinctly  divided  by  great  longitudinal 
valleys  into  parallel  ranges  and  ridges ;  and  these, 
again,  are  serrated  along  their  crests,  or  divided  into 
peaks  by  transverse  valleys.  In  many  cases  this  ideal 
chain  is  far  from  realized,  but  we  have  instead,  a  great 
bulging  of  the  earth's  crust  composed  on  the  surface 
of  an  inextricable  tangle  of  ridges  and  valleys  of  ero- 
sion, running  in  all  directions.  In  all  cases,  however, 
the  erosion  has  been  immense ;  for  the  mountain- 
chains  are  the  great  theatres  of  erosion  as  well  as  of 
igneous  action.  As  a  general  fact,  all  that  we  see, 
when  we  stand  on  a  mountain-chain  —  every  peak  and 
valley,  every  ridge  and  canon,  all  that  constitutes  scen- 
ery—  is  wholly  due  to  erosion."  —  LE  CONTE. 

The  structure  of  mountains  thus  falls  under  two 
heads  :  ( i )  The  internal  structure  and  altitude,  which 
are  due  to  the  action  of  the  subterranean  agencies. 
(2)  The  external  forms,  the  actual  relief,  which  are 
the  product  chiefly  of  the  superficial  agencies  or 
erosion.  The  study  of  mountains  has  shown  that : 
( i )  They  are  composed  of  very  thick  sedimentary  for- 
mations. Thus  the  sedimentary  rocks  have  a  thickness 
of  40,000  feet  in  the  Alleghanies ;  of  50,000  feet  in  the 
Alps ;  and  of  two  to  ten  miles  in  all  important  moun- 
tain-chains. Such  thick  deposits  of  sediments,  as  we 
have  already  seen,  must  be  formed  on  a  subsiding  sea- 
floor,  and  in  many  mountain-chains,  as  in  the  Alle- 
ghanies, the  great  bulk  of  these  sediments  are  still 


194  STRUCTURAL    GEOLOGY. 

below  the  level  of  the  sea.  Again,  thick  sedimentary 
deposits  can  only  be  formed  in  the  shallow,  marginal 
portions  of  the  sea;  and  when  such  a  belt  of  thick 
shore  deposits  yields  to  the  powerful  horizontal  thrust, 
and  is  crumpled  and  mashed  up,  it  is  greatly  shortened 
in  the  direction  of  the  pressure  and  thickened  verti- 
cally, so  that  its  upper  surface  is  lifted  high  above  the 
level  of  the  sea,  and  a  mountain-chain  is  formed  and 
added  to  the  edge  of  the  continent.  We  thus  find  an 
explanation  of  the  important  fact  that  on  the  several 
continents,  but  notably  on  the  two  Americas,  the  prin- 
cipal mountain-ranges  are  near  to  and  parallel  with  the 
coast  lines. 

2.  The  mountain-forming  sediments  are  usually 
strongly  folded  and  faulted,  and  exhibit  slaty  cleavage 
wherever  they  are  susceptible  of  that  structure ;  and 
the  older  rocks,  especially,  in  mountains  are  often 
highly  metamorphosed,  and  are  traversed  by  numer- 
ous veins  and  dikes,  the  infallible  signs  of  intense 
igneous  activity. 

"  In  other  words,  mountain  regions  have  been  the 
great  theatres  —  (i)  of  sedimentation  before  the  moun- 
tains were  formed;  (2)  of  plication  and  upheaval  in 
the  formation  of  the  range ;  and  (3)  of  erosion  which 
determined  the  present  outline.  Add  to  these  the 
metamorphism,  the  faults,  veins,  dikes,  and  volcanic 
outbursts,  and  it  is  seen  that  all  geological  agencies 
concentrate  there."  —  LE  CONTE. 

Since  mountain-ranges  are  great  up-swellings  or 
bulgings  of  trfe  strata,  their  structure  is  always  essen- 
tially anticlinal ;  and  they  sometimes  consist  of  a  sin- 
gle more  or  less  denuded  anticline  (Fig.  38),  the 


PETROLOGY.  IQS 

oldest  and  lowest  strata  exposed  forming  the  summit 
of  the  range.  More  commonly,  however,  the  single 
great  arch  or  uplift  is  modified  by  a  series  of  longitudi- 
nal folds,  as  shown  in  the  section  of  the  Jura  Moun- 
tains (Fig.  21).  Still  more  commonly  the  folds  are 
closely  pressed  together,  overturned,  broken,  and  al- 
most inextricably  complicated  by  smaller  folds,  con- 
tortions, and  slips. 

The  strata  on  the  flanks  of  the  mountains  are  usually 
less  disturbed  than  those  near  the  axis  of  the  range, 
and  are  sometimes  seen  to  rest  unconformably  against 


Fig.  38.  —  Anticlinal  mountain. 

the  latter.  In  this  way  it  is  proved  that  some  ranges 
are  formed  by  successive  upheavals.  But  we  have 
still  more  conclusive  evidence  that  mountains  are 
formed  with  extreme  slowness  in  the  fact  that  rivers 
sometimes  cut  directly  through  important  ranges. 
This  proves,  first,  that  the  river  is  older  than  the 
mountains ;  second,  that  the  deepening  of  its  channel 
has  always  kept  pace  with  the  elevation  of  the  range. 

CONCRETIONS  AND  CONCRETIONARY  STRUCTURE. — 
Folds,  cleavage,  faults,  and  joints  —  all  the  subsequent 
structures  considered  up  to  this  point  —  are  the  prod- 
uct of  mechanical  forces.  Chemical  agencies,  although 
very  efficient  in  altering  the  composition  and  texture 
of  rocks,  are  almost  powerless  as  regards  the  develop- 


iq6  STRUCTURAL    GEOLOGY. 

ment  of  rock-structures ;  and  the  only  important 
structure  having  a  chemical  origin  is  that  named 
above. 

Concretions  are  formed  by  the  segregation  of  one 
or  more  of  the  constituents  of  a  rock.  But  there  are 
three  distinct  kinds  of  segregation.  If  the  water  per- 
colating through  or  pervading  a  rock,  dissolves  a  cer- 
tain mineral  and  afterwards  deposits  it  in  cavities  or 
fissures,  amygdules,  geodes,  or  veins  are  the  result.  If 
the  mineral  is  deposited  about  particular  points  in  the 
mass  of  the  rock,  it  may  form  crystals,  the  rock  be- 
coming porphyritic  ;  or  it  may  not  crystallize,  but  build 
up  instead  the  rounded  forms  called  concretions,  the 
texture  or  structure  of  the  rock  becoming  concretionary. 
A  great  variety  of  minerals  occur  in  the  form  of  con- 
cretions, but  this  mode  of  occurrence  is  especially 
characteristic  of  certain  constituents  of  rocks,  such  as 
calcite,  siderite,  limonite,  hematite,  and  quartz.  Con- 
cretions may  be  classified  according  to  the  nature  of 
the  segregating  minerals ;  and  in  each  class  we  may 
distinguish  the  pure  from  the  impure  concretions.  A 
pure  concretion  is  one  entirely  composed  of  the  seg- 
regating mineral.  Most  nodules  of  flint  and  chert, 
quartz,  geodes,  concretions  of  pyrite,  and  many  hollow 
iron-balls  are  good  illustrations  of  this  class.  In  all 
these  cases  the  segregating  mineral  has  been  able  in 
some  way  to  remove  the  other  constituents  of  the  rock, 
and  make  room  for  itself.  But  in  other  cases  it  has 
lacked  this  power,  and  has  been  deposited  between 
and  around  the  grains  of  sand,  clay,  etc. ;  and  the 
concretions  are  consequently  impure,  being  composed 
partly  of  the  segregating  mineral,  and  partly  of  the 


PETROLOGY.  197 

other  constituents  of  the  rock.  The  calcareous  con- 
cretions known  as  clay- stones  are  a  good  example  of 
this  class,  being  simply  discs  of  clay,  all  the  minute 
interstices  of  which  have  been  filled  with  segregated 
calcite.  The  solid  iron-balls  are  masses  of  sand  filled 
in  a  similar  manner  with  iron  oxides. 

Concretions  are  of  all  sizes,  from  those  of  micro- 
scopic smallness  in  some  oolitic  limestones  up  to  those 
twenty-five  feet  or  more  in  diameter  in  some  sand- 
stones. 

The  point  of  deposition,  when  a  concretion  begins  to 
grow,  is  often  determined  by  some  concrete  particle, 
as  a  grain  or  crystal  of  the  same  or  a  different  mineral, 
a  fragment  of  a  shell,  or  a  bit  of  vegetation,  which  thus 
becomes  the  nucleus  of  the  concretion.  The  ideal  or 
typical  concretion  is  spherical ;  but  the  form  is  influ- 
enced largely  by  the  structure  of  the  rock.  In  porous 
rocks,  like  sandstone,  they  are  frequently  very  perfect 
spheres ;  but  in  impervious  rocks,  like  clay,  they  are 
flat  or  disc-shaped,  because  the  water  passes  much 
more  freely  in  the  direction  of  the  bedding  than  across 
it ;  while  the  concretions  in  limestones,  the  nodules  of 
flint  and  chert,  are  often  remarkable  for  the  irregularity 
of  their  forms.  In  all  sedimentary  rocks  the  concre- 
tions are  arranged  more  or  less  distinctly  in  layers 
parallel  with  the  stratification,  which  usually  passes 
undisturbed  through  the  impure  concretions.  Many 
silicious  and  ferruginous  concretions  are  hollow,  ap- 
parently in  consequence  of  the  contraction  of  the  sub- 
stance after  its  segregation ;  and  the  shrinkage  due  to 
drying  is  still  further  indicated  by  the  cracks  in  the 
septaria  stones.  The  hollow,  silicious  concretions  are 


198  STRUCTURAL    GEOLOGY. 

usually  lined  with  crystals  (geodes),  while  the  holiow 
iron-balls  frequently  enclose  a  smaller  concretion. 
Rocks  often  have  a  concretionary  structure  when 
there  are  no  distinct  or  separable  concretions.  And 
the  appearance  of  a  concretionary  structure  (pseudo- 
concretions)  is  often  the  result  of  the  concentric  de- 
composition of  the  rocks  by  weathering,  as  explained 
on  page  13. 

SUBSEQUENT  STRUCTURES  PRODUCED  BY  THE  SUPER- 
FICIAL OR  AQUEOUS  AGENCIES.  —  The  superficial  agen- 
cies, as  we  have  seen  in  the  section  on  dynamical 
geology,  are,  in  general  terms,  water,  air,  and  organic 
matter.  Geologically  considered,  the  results  which 
they  accomplish,  may  be  summed  up  under  the  two 
heads  of  deposition  and  erosion  —  the  formation  of 
new  rocks  in  the  sea,  and  the  destruction  of  old  rocks 
on  the  land.  In  the  role  of  rock-makers  they  produce 
the  very  important  original  structures  of  the  stratified 
rocks ;  while  as  agents  of  erosion  they  develop  the 
most  salient  of  the  subsequent  structures  of  the  earth's 
crust  —  the  infinitely  varied  relief  of  its  surface.  As  a 
general  rule,  to  which  recent  volcanoes  are  one  im- 
portant exception,  the  original  and  subterranean  struc- 
tures of  rocks  are  only  indirectly,  and  often  very 
slightly,  represented  in  the  topography;  for  this,  as 
we  have  seen,  is  almost  wholly  the  product  of  erosion. 
Therefore,  what  we  have  chiefly  to  consider  in  this  sec- 
tion is  to  what  extent  and  how  erosion  is  influenced 
by  the  pre-existing  structures  of  rocks. 

Horizontal  or  very  slightly  undulating  strata,  espe- 
cially if  the  upper  beds  are  harder  than  those  below, 
give  rise  by  erosion  to  flat-topped  ridges  or  table- 


PETROLOGY. 


199 


mountains  (Fig.  39).  But  if  the  strata  be  softer  and 
of  more  uniform  texture,  erosion  yields  rounded  hills, 
often  very  steep,  and  sometimes  passing  into  pinnacles, 
as  in  the  Bad  Lands  of  the  west.  Broad,  open  folds, 
as  we  have  seen,  give,  normally,  synclinal  hills  and 
anticlinal  valleys  (Fig.  22),  when  the  erosion  is  well 
advanced.  But  in  more  strongly,  closely  folded  rocks 


Fig.  39.  —  Horizontal  strata  and  table-mountains. 

the  ridges  and  valleys  are  determined  chiefly  by  the 
outcrops  of  harder  and  softer  strata,  as  shown  in  Fig. 
40,  the  symmetry  of  the  reliefs  depending  upon  the 
dip  of  the  strata.  This  principle  of  unequal  hardness 
or  durability  also  determines  most  of  the  topographic 


Fig.  40.  —  Ridges  due  to  the  outcrops  of  hard  strata. 

features  in  regions  of  metamorphic  and  crystalline 
rocks,  in  which  the  stratification  is  obscure  or  wanting. 
The  boldness  of  the  topography,  and  the  relation  of 
depth  to  width  in  valleys,  depends  largely  upon  the 
altitude  above  the  sea ;  but  partly,  also,  upon  the  dis- 
tribution of  the  rainfall,  the  drainage  channels  or  val- 
leys being  narrowest  and  most  sharply  defined  in  arid 
regions  traversed  by  rivers  deriving  their  waters  from 


200  STRUCTURAL    GEOLOGY. 

distant  mountains.  That  these  are  the  conditions 
most  favorable  for  the  formation  of  canons  is  proved 
by  the  fact  that  they  are  fully  realized  in  the  great 
plateau  country  traversed  by  the  Colorado  and  its 
tributaries,  a  district  which  leads  the  world  in  the  mag- 
nitude and  grandeur  of  its  canons.  But  deep  gorges 
and  canons  will  be  formed  wherever  a  considerable 
altitude,  by  increasing  the  erosive  power  of  the  streams, 
enables  them  to  deepen  their  channels  much  more 
rapidly  than  the  general  face  of  the  country  is  lowered 
by  rain  and  frost.  This  is  the  secret  of  such  canons 
as  the  Yosemite  Valley,  and  the  gorge  of  the  Columbia 
River,  and  probably  of  the  fiords  which  fret  the  north- 
west coasts  of  this  continent  and  Europe.  For  a  full 
description  and  illustration  of  the  topographic  types 
developed  by  the  action  of  water  and  ice  upon  the 
surface  of  the  land,  and  of  the  various  characteristic 
forms  of  marine  erosion,  teachers  are  referred  to  the 
larger  works  named  in  the  introduction,  especially 
Le  Conte's  Elements  of  Geology,  and  to  the  better 
works  on  physical  geography.  We  will,  in  closing  this 
section,  merely  glance  at  some  of  the  minor  erosion- 
forms,  which  are  not  properly  topographic,  but  may  be 
often  illustrated  by  class-room  and  museum  specimens. 
Mere  weathering,  the  action  of  rain  and  frost,  develops 
very  characteristic  surfaces  upon  different  classes  of 
rocks,  delicately  and  accurately  expressing  in  relief 
those  slight  differences  in  texture,  hardness,  and  solu- 
bility, which  must  exist  even  in  the  most  homogeneous 
rocks.  Every  one  recognizes  on  sight  the  hard,  smooth 
surfaces  of  water-worn  rocks.  They  are  exemplified 
in  beach  and  river  pebbles,  in  sea-worn  cliffs,  and 


PETROLOGY.  201 

where  rivers  flow  over  the  solid  ledges.  The  pot-hole 
(page  17)  is  a  well-marked  and  specially  interesting 
rock-form,  due  to  current  or  river  erosion. 

Ice  has  also  left  highly  characteristic  traces  upon 
the  rocks  in  all  latitudes  covered  by  the  great  ice- 
sheet.  These  consist  chiefly  of  polished,  grooved,  and 
scratched  or  striated  surfaces,  the  grooves  and  scratches 
showing  the  direction  in  which  the  ice  moved. 

The  organic  agencies,  as  already  noted,  accomplish 
very  little  in  the  way  of  erosion,  especially  in  the  hard 
rocks,  but  the  rock-borings  made  by  certain  mollusks 
and  echinoderms  may  be  mentioned  as  one  unimpor- 
tant but  characteristic  form  due  to  organic  erosion. 


APPENDIX 


The  following  collections  are  especially  pre- 
pared and  arranged  for  use  with  this  text : 

2 1  Opalized  wood 
*22  Gypsum 
*23  Calcite 
°24  Dolomite 

25  Siderite 
*26  Hornblende 
°27  Pyroxene 
*28  Muscovite 

29  Biotite 
*3<D  Orthoclase 
°3i  Albite 
*32  Labradorite 
*33  Kaolinite 

34  Talc 
*35  Serpentine 
°36  Chlorite 

37  Glauconite  (Green  Sand) 

38  Chrysolite 

39  Garnet 

40  Pyrite 


Weathering 

i  Diabase 

*2        "        weathered 
*3        "        disintegrated 
°4  Felsite :  Angular  fragment 
°5        "       Water      rounded 
pebble 

Formation  of  Coals 

*6  Peat 

°;  Lignite 
8  Bituminous 

*9  Cannel  coal 
°io  Anthracite 
°n  Native  coke 

Rock-forming  Minerals 

*i2  Graphite 
°I3  Halite 
*i4  Limonite 
*I5  Hematite 
*i6  Magnetite 

17        "  Lodestone 

*i8  Quartz:  Glassy 

19  "          Flint 

20  "         Chert 


Sedimentary  and  Metamorphic 
Rocks 

*4i  Conglomerate  :  Breccia 
*42  "  Pudding- 

stone 


APPENDIX 


203 


*43  Sand :  Quartz 

°44      "       Magnetite 

*45  Sandstone:  Ferruginous 

46  "  Calcareous 

47  "  Arkose 
*48  Quartzite 

49  Clay:  Boulder 
°5<D      "       Fire 
*5i  Shale 
*52      "      Carbonaceous 

53  Slate:  Roofing 

54  "       Flagstone 

55  Porcelainite 

56  Tripolite 

°57  Siliceous  Tufa 

58  Novaculite 
°59  Asphaltum 
°6o  Oil  Sand 

*6i  Limestone:  Fossiliferous 
*62  Coquina 

#63  "  Chalk 

64  "  Crystalline 

%5  "  Compact 

66  "  Hydraulic 

67  Calcareous  Tufa 

68  Dolomite 

69  Rock  Salt 

°70  Phosphate  Nodule 
*7i  Gneiss :  Granitoid 
*72         "        Micaceous 
73        "        Hornblendic 


°74  Norite  :  Hypersthenite 
*75  Schist :  Mica 

76  "        Hornblende 

77  "        Talc 

78  "        Chlorite 

79  Amphibolite 

80  Soapstone 

8 1  Verd    Antique    (Serpen- 

tine) 

Igneous  Rocks 

*82  Granite :  Binary 
83         "        Muscovite 
*84         "        Biotite 

85  "        Hornblendic 

86  "        Red 
*87  Syenite 

88       "       Elaeolite 
*8g  Diorite 
*9O  Diabase :  Trap 
*9i  Rhyolite 

92  Trachyte 
*93  Obsidian 

94  Pumice 
°95  Petrosilex 
*96  Andesite 
*97  Basalt^- 

98      "       vesicular  Lava 
*99  Melaphyr:  Amygdaloidal 
°ioo  Volcanic  Tuff 


Collection  No.  Fi.  Entire  list  of  100  museum  size  spe- 
cimens (3%x4X)>  numbered,  labelled  and  mounted 
on  blocks  or  in  improved  trays,  for  museum  display 

and  laboratory  work $40.00 

(The  same,  labelled  but  unmounted,  $30.00) 


204  APPENDIX 

Collection  No.  F2.  Same  as  above,  but  small  museum 
size,  mounted  in  improved  trays  (2^x3^)  ....  $25.00 

Collection  No.  fj.  Same  as  Fz,  but  hand  size  speci- 
mens (2x2) 12.50 

Collection  No.  F4.  80  specimens,  omitting  those 
marked  (°),  in  individual  trays  (2^x1^)  and  two 
cloth-board  cases,  numbered  to  correspond  with  ac- 
companying printed  list  (no  labels) 5.00 

Collection  No.  F^.  40  specimens  marked  (*),  mounted 
as  collection  F4 2.50 

Collection  No.  F6.  100  pupils'  fragments  (ixi),  num- 
bered, in  paper  bags.  (Single  collection  $1.25.)  In 
lots  of  5  or  more,  each i.oo 

Collection   No.   Fj.    80   pupils'  fragments    (like   F6). 

(Single  $1.00.)     In  lots  of  5  or  more,  each  ....  .75 

Collection   No.   F8.     40  pupils'  fragments    (like  F6). 

(Single  5oc.)     In  lots  of  5  or  more,  each .40 

Collection  No.  Fg.  25  museum  size  specimens,  illus- 
trating structure,  faults,  stratification,  etc.  Mounted 
and  labelled 10.00 


For  further  information  or  in  ordering,  address 

WARD'S   NATURAL  SCIENCE  ESTABLISHMENT 

84-102  College  Ave.,  Rochester,  N.  Y. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 
EARTH  SCIENCES  LIBRARY 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


LD  21-40m-5,'65 


General  Library 
University  of  California 


% 


c 


5.30 


